This is a missable achievement. You need the achievement ''The Final Instance'' to face the Absolute Conqueror and unlock this one.
I was BR121 when I defeated him.
The method in the other guide almost worked for me but not quite. The reason why my BR was high is because I didn't really understand how to upgrade my skills until very late in the game. I think I learned my first weapon arts with Baulson and Jager between BR105-110.
I was lucky but by using this method I got him on the first try.
Single union method:
The key here is to use characters that can use weapon arts, Remnants arts and Unique arts, you want a team composed of your five strongest leaders with a decent amount of HP. I had over 9000HP when I did it.
Remember that when your are using unique arts (Omnistrike, Protection, Hundred Flowers), you have a very good chance to dodge every attacks. don't be afraid to use them even if your health is low, if you are lucky your team will dodge, but watch out for end of turn additional attacks, those can be still be dangerous, my health dropped below 500HP after one of those.
My team was:
Rush (Omnistrike, Protection), Dave (Ex Machina), Baulson (Heaven's Door), Emmy (Hundred Flowers), Jager (Beowulf, Schiavona)
- Do not use summons, it might be a good tactic when using multiple unions but it's probably a bad idea to use it with a single union since it could trigger an additional attack that could hit your only union.
Against the first two bosses, start with the big one. With the amount of HP and the strengh of your team, it shouldn't be a problem to take him out without having to heal yourself. Still if you see your HP getting low, just take a turn to ''Keep Your HP Up''. If you can, you should use this just before defeating the second boss to make sure you have enough HP for when the Absolute Conqueror arrives.
Now time for the Absolute Conqueror, use weapons/remnants and unique arts each time you can. If your health is low be careful using weapons and remnants arts, you might be better off healing yourself before using those.
Keep using your most powerful arts, if you don't get good options, use ''Play it by the ear''. Take time to heal yourself when you fear your HP is getting too low. Depending on the situation, if your HP is very low, you might want to get out of Deadlock to heal your team but try to avoid it so you don't lose an entire turn of attacks.
If the Conqueror slams you with status ailments or kills someone in your union, clear your ailments as quickly as possible, unless you are given the chance to hit him with some of your most powerfull arts, then wait for the good opportunity to do it but be careful.
When you are getting near the end of the battle, keep your HP as high as you can because you might have to absorb the Conqueror's berserk attacks and some other powerful attacks at the end of turn. A good way to avoid those is to use unique arts if you can.
I think that while using this method, the Conqueror's most dangerous attack is probably Obliterate III, if it hits, your union will die. With some luck, he won't do this attack or you might be able to dodge this using a unique art like I did.
Here's a video (credits to TheJRPGgamer) using the single union method with a different team: