Getting the "Stay on Target" achievement is tricky, because you need to have an AI empire use a colossus against you. However, with the right galaxy settings you can ensure that this happens and not waste time waiting for the AI to construct one.

• Galaxy size: Huge (1000 stars)
  
• AI empires: 0
  
• Fallen empires: 5 (or 4 if you don't have Synthetic Dawn)
  
• Marauder empires: 0
  
• Tech/Tradition cost: 0.25
  
• Crisis strenghth: off
  
• End-game start year: 3000

The other options don't really matter that much, so adjust as you want. For civics I recommend Fanatical Purifiers with Militarist ethos. The idea is to have maximum fire rate, so you can quickly shred the FE fleets.

Important: After you generate a new game, exit to main menu and open the save game file in a text editor. If the save game file contains the text

design="SHIP_COLOSSUS_SECTION"

this means that one of the FEs has spawned with a colossus. If that text is not there, then you will most likely not fight against a colossus this game, so you need to abandon it. Keep making new games until you create one where a colossus is spawned at the beginning.

Since all of your enemies are sleeping Fallen Empires, you are free to build and expand your empire with no danger. You need to find the FE which has the colossus. Either build a sentry array to locate it or find that FE with your science ship (which should have good sensors). It's important to have intel on the colossus before you proceed. It's also important to have good intel around the FE where the colossus is.

Once you have identified the FE with the colossus, then this is obviously the FE which you will fight. The next step is determine which is the most likely planet that the AI will attack after you declare war on the FE (or you bait the FE to declare on you). This can be difficult to determine, but as a general rule, colonize a planet which is very close to the FE. This is most likely the planet which the FE will attack. However be mindful of wormholes / gateways, you need to take those into account!

Once you have colonized a planet close to the FE, you have to fortify it like crazy. Do not upgrade the starbase in the system, because it will be used against you. Only fortify the planet! Build a military academy, a planetary shield generator and fortresses on all other tiles. The idea is that this planet will be so well-defeneded that the FE will not be able to conquer it and would therefore use the colossus to wipe it.

You should also construct a gateway in that system as close to the planet as possible. The idea is that you will be able to use the gateway with all your fleets to ambush the enemy FE. Of course, your own fleets should be sitting on top of another gateway somewhere else in your empire.

Important: Savescum!!!

It's very important to savescum here. You should have a backup of your savegame before you declare war on the FE or you bait the FE in declaring on you. This can easily go wrong and the colossus has a big chance of doing an emergency FTL. In that case you might not get the achievement.

After you have fortified the planet and you have build several massive fleets (It's a good idea to have at least twice the fleet power of the FE) you can now prepare for the engagement.

You should have at least one Titan which has the FTL offensive aura on. This would double the time it takes for the enemy to do an emergency FTL jump. Even with this, I only got the achievement on my second try (which is why it is important to savescum).

Ideally you should declare a rivalry or insult the enemy FE so that it declares war on you. This would provide you with a +33% increase fire rate because of the defensive war bonus perk. But even if you can't bait the FE, you can still do it, by just declaring war on them. Make sure that you have a massive fleet and before you engage:

• Use the unity ambition that buffs the fire rate
  
• Use the No Retreat war doctrine that buffs the fire rate
  
• Have a good admiral with buffs to fire rate
  
• Basically do everything you can to buff fire rate / damage of your fleet.

After war is declared by either party, the FE should attack the bait planet which you have fortified like crazy. In my case, the FE had 2 fleets. The first (bigger one) was sent to the planet along with armies to capture it and the second fleet was holding back protecting the colossus in a nearby system. As soon as the bigger fleet started bombarding the planet, I jumped on them from the gateway and shred the fleet along with the transports. Then I moved out of the gateway back into the other end of the gateway (which the FE cannot access).

Then the FE sent the remaining (smaller) fleet along with the colossus in order to wipe the planet.

Important: You should jump on the colossus as soon as it starts charging, not before.

With good preparation, inspecting the save game file, proper build up and (sadly) savescumming, this achievement is actually not that difficult to get and it is rather straightforward.