In order to get the Made It! achievement, you have to prevent the game to wrongly flag Gilgamesh's appearance on its last area, the Interdimensional Rift. This is a missable achievement, due to some things I'll briefly explain - there are two main points of no return.

This seems to be a quite common achievement, but if you make some mistakes not only you won't be able to get it, but you'll probably have to replay the game in its entirety and if you played FFV without achievements on SNES, GBA and so on you won't probably even remember all these things.

The first point of no return is in the end of World 2, as you'll read below, because if you kill the 4th Gilgamesh form, having 55000 HP, ExDeath won't basically send him over to the Rift and Gilgamesh won't appear in the Third World.

The second point regards the Necrophobe fight; of course, if the first point is missed you'll have no chance here - you'll fight Necrophobe alone before the final boss. Though, since Necrophobe has only 44000 HP and you can be overleveled, you may kill him without letting Gilgamesh appear and do the dirty job for you, preventing you to get the achievement - the boss will transform into a save point, and you'll be again screwed.

Let's see briefly the two sections - I hope in the near future to make them better-looking and such.

Here are the times you have to fight Gilgamesh - they're all unmissable, but your behavior in the fourth one will decide if you're getting or not the achievement:

1) Inside ExDeath's castle in the Second World, at the very beginning. Here, you play with Galuf only. You have to deal 1500 HP of damage, and he'll flee.

2) On the Big Bridge, after fighting several unmissable (and random) battles. He has two forms: after dealing some 4000 damage, he'll cast on himself Shell, Haste and Protect - actually, here you are required only to kill him regardless of damage dealt and quickness. 6500 HP overall.

3) Always on the Second World, aboard Xezat's Fleet east of ExDeath's castle (which is still now closed by the barriers). Gilgamesh will be on the southernmost tip of the main ship. Here, Gilgamesh has 9000 HP, but when he is on 6000 or less he will summon Enkidu, a fellow boss of 4000 HP. Again, you do not even need to make Enkidu appear here - you just need to kill Gilgamesh.

4) Finally, entering ExDeath's castle at the end of World 2. On the 11th floor, you must open the empty chest which can be found in the center of the room; upon going north, Gilgamesh will appear and start a battle. Here's the FIRST breakdown. What you have to do here is to bring him down of about 13000 HP. He has 55000 HP totally, and if you kill him quickly you'll never see him again. After dealing him some damage, he'll start talking every turn. If you are overleveled, you must be wary and keep dealing 1000 or 2000 HP of damage before he transforms. This transformation is MANDATORY. The main thing is that if you kill him NOW, ExDeath won't come and send him to the Void, the last area of the game. So, AFTER his transformation, you must wait that ExDeath comes and kills him, actually warping him to his last destination. Do not kill Gilgamesh at any cost, keep on dealing small quantities of damage!

5) If you did everything correctly, on World 3 you will enter the Interdimensional Rift - the last area before the final boss, after having collected all the 12 legendary weapons and such. Upon entering the area, you'll see on the southeast Gilgamesh himself, on the map. Remember - if ExDeath hadn't "killed" him for you before, he won't appear. This battle is another strange one, like all the Gilgamesh fights! As always you can steal some Genji equipment from him - but we aren't here for equipment issues. He has 37000 HP, but you're not obliged to finish him off; you can steal from him or simply resist his attacks and counter; at some point he'll start talking, and yet again he will flee.

6) The Necrophobe fight. At the end of the Rift, on the west side of the last area, you'll find a green (on Steam) light - approaching it will trigger Necrophobe, with his four protecting Barriers. Be really wary here if you are overleveled. You must kill the Barriers first (with Zeninage or with whatever you have), and then you'll have to dispose of Necrophobe itself. The boss has 44000 HP, and he will trigger Gilgamesh ONLY when his HP are LESS THAN 10000 - so, if you kill him directly, you'll still avoid the achievement. Keep pummeling with 1000-2000-HP attacks and, when he's almost done, Gilgamesh will appear, he'll talk and he'll sacrifice himself killing Necrophobe, regardless of how many hit points he still has.

That's the point in which you'll get the achievement, and a save point will appear below you before the final fight!