This achievement is most likely going to be the last one you earn in the game:
Defeating the Arch-Illager on the hardest difficulty in the game, for the apocalypse difficulty you have two sub difficulties for the fight.
V) with a recommended power level of 91
VI) with a recommended power level of 97
I managed to do this with a team of 2 with an :
individual power rating of 99
team power rating of 101
in terms of gear, the best advice I can give is, always equip the highest power gear you have, even if you hate the piece of equipment equip it this increases your power level which increases the loot range found at the bottom of the mission select screen. for example
me being power level 99 apocalypse VI has a loot range for new gear of 100 -107
however if i lower my power rating it drops to 88-95
i would defo recommend that all members of your party are above the 100 power level range. and also fighting the boss on sub difficulty 5 for a lower power recommendation of 91
(hide spot for the boss) - there may be a hiding spot for the boss fight that teammates can use to avoid damage (not completely ) as soon as you hit the final jump pad to be bounced to the boss platform, you'll notice a tiny ledge behind you with a few steps heading back (3-4 steps) if one teammate sits on the very bottom step at the top right wall... there is an increased chance of avoiding damage however sometimes during the second phase fight. the "ender beams" (big creatures that pop out of the floor and shoot the lasers) they may randomly spawn and shoot in your direction. be prepared to move out of the way, heal and return if needed.