Poetry Critic Achievement
Defeat The Magus.Story-related; cannot be missed.
This fight is the entirety of Chapter 16. This fight is again simple but a bit different since you don't actually fight Magus at all. You'll fight a pretty large swarm of enemies (they aren't infinite), get a cutscene, and then repeat with another wave. Each time you have dialogue options, make the obvious choice to antagonize him. For the waves of enemies, they can be very overwhelming, especially the slugs that jump onto you and do massive damage before you can push them off. Groot will automatically revive you every couple minutes with his final ability, so don't bother using it, just let it trigger if you die. Between his ability recharging, you should be able to use a huddle to refill your health once too. Since there are no really strong enemies, just tons of regular ones, stick to ally abilities that hit as many as possible. Rocket's starting grenade is amazing for this, as it kills almost every enemy instantly and recharges very quickly.
After the second wave of enemies are defeated, you'll get a simple quick time event, and then this achievement will unlock.