First things first: Yes, there is an achievement for beating the game on "Hard" difficulty. You can change the difficulty from normal to hard at any point in the game & only need to beat the final boss with hard selected, though. So feel free to start a game on Normal difficulty, if you're more comfortable that way.

While the game separates the story into 10 chapters, it doesn't provide an actual chapter select; not even after finishing the story. What it does provide though, are 30 available save-slots.
So I highly recommend to create your own, artificial chapter select, by keeping at least 1 save per chapter.
Which means, after the first few hours it should look something like this:

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With your artificial chapter select in place, there are 2 different kinds of missable achievements within Crisis Core Reunion.

There are several self-contained achievements, at least one per chapter, which you can obtain easily by loading a save from your chapter select, playing a few minutes while skipping all cutscenes and fulfilling the achievement's requirement right away.
Those are located in Part 2 of this guide and have no further implications for other achievements.
Even if your "Main Save" is already further into the game, you don't have to reset any progress.

And then there are the long term consequences. Some easily missable events from chapter 3 onwards, which seem insignificant, but will stop you from obtaining some achievements a few dozen hours later; deep into the final chapter. You'll find these events in Part 1 just below. Depending on which you may have missed, you might have to forfeit all your progress beyond those parts and do everything again... - sorry.

If you want to go as spoiler-free through the game as possible, while still guaranteeing 100% achievements, create your own chapter select and closely follow Part 1 below.

The first event you absolutely have to get right is at the very beginning of chapter 3.
Feel free to jump into the game right now and come back here, once you've unlocked the achievement for completing chapter 2.

With your own artificial chapter select in place, you'll only need to know about the following multi-chapter achievements during your continuous main playthrough:


The game has a total of 300 optional "Missions", which you can access at the various save points within the game.
Around 2/3 of them are intended to be played along the Main Story to get good equipment and to level Zack. The last third quickly becomes more difficult than the Main Story's final area and is basically this game's "Post Game".
Along completing these missions, you'll unlock several shops, mail senders and "DMW images", aka summons. You need to get ALL of these for the achievements listed above.

But during the story, there're:

• 1 missable summon / DMW image
  
  • Phoenix (chapter 8)
• 5 missable shops
  
  • Research Dept. QMC+ (chapter 9)
    
  • The Happy Turtle (mission 4-3-6, see below)
    
  • Sec.5, 6 & 8 Materia Shops (mission 6-2-1, 6-2-3 & 6-2-6, see below)
• 8 missable mail senders
  
  • 3x Fan Clubs (chapter 3)
    
  • Reporter (chapter 5)
    
  • "The Silver Elite" fan club (chapter 5)
    
  • Zacks Fan Clubs (chapter 6 into 7)
    
  • Wonder Hunter (chapter 8 into 9)
    
  • Shop News (mission 6-2-1, see below)
• and 54 missable missions (16 mission unlocks + 38 follow-up missions)
  (Midgar free roam NPCs; chapter 7 at the latest)
  
  • 2-1-1 to 2-1-6
    
  • 4-3-1 to 4-3-6
    
  • 1-2-1
    follow-ups:
    
    • 1-2-2 to 1-2-6
  • 7-1-1
    follow-ups:
    
    • 7-1-2 to 7-1-6
  • 8-2-1
    follow-ups:
    
    • 8-2-2 to 8-2-6
  • 6-2-1
    follow-ups:
    
    • 6-2-2 to 6-2-6
      
    • 6-4-1 to 6-4-6
      
    • 6-5-1 to 6-5-6
      
    • 6-6-1 to 6-6-6

If you're missing any of them by the time you're in chapter 10, you'll have to reload and play through some chapters again...

You only need 1 mail from each sender. You don't need to collect all mails.
For the mission-exclusive mail senders, you'll only have to play mission 6-2-1 (in itself also a missable from chapter 7) and all 12 missions from 8-4-1 to 8-5-6.

The final shop unlocks upon completing mission 9-5-4, which is among the 10 hardest missions of the game, so you really want to do all shops on your 100% missions run.

The only permanently missable piece of Genji armor is rewarded for achieving the 100% progress for all DMW... but the other 3 pieces are also very endgame stuff, so don't miss that Phoenix materia.

If you haven't already done so during chapter 2 free roam, you can still join the original 3 fan clubs in Sector 8:

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This chapter starts with a free roam through Midgar.
Start by going to Sector 8 - Fountain and locate the "Reporter".
Make absolutely sure to let him go, as that's the only way to unlock him as a mail senders.

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Once that's done, head down to Slums Park to join the next fan club. This one needs to be joined in this chapter for her to send you mails. She has a test for you, but you have infinite tries to pass.

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During chapter 6, you'll get to play this minigame:

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Successfully stopping all 30 enemies will unlock an achievement from Part 2 below, but isn't necessary for your continous playthrough. I'm mentioning it anyway, as you'll have to trigger the following interaction immediately afterwards. This is a requirement for a fan club (mail sender) in chapter 7:

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Chapter 7 offers the final free roam of Midgar. You may already have done most of the stuff mentioned for this chapter, as almost all sidequest have been available in earlier chapters as well.
Make absolutely sure to complete all the stuff mentioned below for this chapter, before you hit the point-of-no-return:

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Let's start by making good on the chapter 6 interaction by joining the final fan club , to add it to our list of mail senders:

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Doing this will also unlock the achievement:



At this point, we'll have to collect access to a few dozen of the 300 missions.
On LOVELESS Avenue, you'll be able to start a side-quest, which requires you to find 6 Wutai spies. Each of them unlocks 1 out of the 300 missions (4-3-1 to 4-3-6), which you don't have to complete at this point though. Mission 4-3-6 will also unlock a shop, once completed.
You can start the quest here:

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Completing this is rather easy, as these spies are completely incompetent. You break their disguise by simply talking to them 3 times.
The order of their locations is: Loveless Avenue, Slum Market, Shinra Building Entrance (upper floor; delayed entrance from elevator), Sector 8 - Fountain, Exhibition Room, Slum Park.

Doing this will also give you a flower wagon part and unlock the achievement:



Next up, the only group of missions you absolutely have to clear right away (2-1-1 to 2-1-5), as the sixth location below only shows up after you've completed the first 5.

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And 3 more sets of 6 missions each (1-2-1 to 1-2-6, then 7-1-1 to 7-1-6 and finally 8-2-1 to 8-2-6).
You only need to unlock the first mission each, as the other are follow-ups.
You don't have to complete these missions yet, but it's recommended, as reporting 8-2-6 to the quest giver awards the "Master Mako Stone Miner" achievement and reporting 1-2-6, 2-1-6 & 7-1-6 all award the remaining 3 wagon parts for the "Midgar Full of Flowers" achievement.
Both of these achievements are technically part of the self-contained list though, so you CAN skip the actual completion of them for now and come back later via your artificial chapter select.

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And the final mission you have to accept right now (6-2-1). This one has a total of 23 follow-up missions, along which you also unlock 1 mail sender and 3 shops. There are no further benefits for completing these missions during chapter 7 though, so feel free to postpone them until you start your actual mission grind.

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Right as you enter the village of Nibelheim, start the quest for Nibelheim's Seven Wonders:

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Make sure to complete as many wonders as you can, without progressing the main story.
The first wonder unlocks the only missable summon and the seventh wonder, which is part of early chapter 9, adds another mail sender.

The seventh wonder will also unlock the achievement:



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This chapter contains the only missable shop... inside a chest...

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And with this, you're done with everything that absolutely HAS to be done on the save file, which you'll use to complete the 300 missions.
Good Luck from here on out!

As mentioned in the basics: All the achievements in this section are missable, but can easily be obtained via the artificial chapter select.


Complete all 16 encounter within Wutai's Fortress, including the 5 hidden encounter for the towers and rotating walls:

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Mark all 5 items for later (3 in village, 2 outside), continue until you have to cut the 10 shells (beware of the camera shift for the tenth) and then collect the 5 items during the countdown.

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Right at the start, enter the Training room and complete all of Hojo's tests. Continue with the story until you have to help the people on SOLDIER Floor and in LOVELESS Avenue.

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First half of Chapter 4.
After you've entered the Slums Market with Aerith and got your pocket picked, do NOT interact with Aerith, until you've talked with all 3 vendors, waited for the girl as often as the game lets you (10x), and traded as many potions to the boy, until Zack refuses to continue (5x).
Now talk to Aerith, catch Bruno and make sure to suggest, that you'll buy the medicine yourself.
After you've got the boy's money back, make another round on the market.
Guess the correct on the first try (the number that both Aerith and boy recommend), collect all 20 materia within 15 seconds, so that he calls you "godlike" and blend the perfume perfectly 3 times in a row. Yes, 3 times. Save and reload in-between to be safe.
Now continue the story until Bruno gives you his Steal materia. If he calls you a "good couple" - congratz. If not, reload the save and give Aerith another perfect perfume.

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Second half of chapter 4.
Continue until you've spoken to Hojo exactly once. This enables 4 interactions with the machines in the room. Stop the Mako Recovery Unit after exactly 5 seconds (infinite tries), enter the test chamber and give the answers: "A SOLDIER operative like Sephiroth." -> "State-of-the-art science and technology." -> "A person unafraid to challenge Shinra" and use the Steal materia to obtain and use all 6 keys.

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Self-explanatory:

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Stop all 30 enemies in time:

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If you haven't already done so in previous chapters, complete mission 8-2-1 to 8-2-6 and report back with the scientist in the Materia room, and head into the Training room for some squat competition:

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Now head to Sector 8 - Fountain and talk to both available Fan Club NPCs. For the money problem questline, head north to LOVELESS Avenue and talk to the second Genesis Fan Club there. For the boy's questline, head to the train station and talk some sense into his mother. Report back near the Fountain for the achievement.

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Complete the missions 1-2-6, 2-1-6 and 7-1-6 (unlocked through free roam NPCs, see Part 1 ch. 7 above) to grab 3 wagon parts and find all 6 Wutai spies for the 4th. Beat all 4 opponents in the physical fitness test for the parts of third wagon.
Continue the story until the point-of-no-return prompt, pick "Wait, I need a little more time!", and go back to Aerith, to build the 2 bonus flower wagons.

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In Nibelheim, complete the Sniper part without letting a single machine escape. You must engage with each individual sniper. Grind them up with normal combat encounters, if you need it.
In Gongaga, play until the Genesis Cutscene at the dead end of the area. Within that area, complete 4 battles (don't ask me why, I don't get it either) and head towards the only zone-transition of the area. This will trigger the hidden waterfall minigame (infinite tries; 4 battles each), in which you need to grab all 10 chests without touching the monsters.

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Within the final underground area, read all 4 tombstones with the LOVELESS Acts and head south into Lake of Oblivion. Interact with all 5 pillars of light and defeat the enemies they spawn. This will unlock the path to the Cage.

After that make sure to set your game difficulty to "Hard" before you approach the final boss.
Good Luck!