Squid Kicker Achievement
Defeat the Dweller-in-Darkness.Story-related; cannot be missed.
At the end of Chapter 4 you will fight the Dweller-in-Darkness. This fight has two phases.
The first phase involves it using its tentacles to try to smash you, two at a time. Open fire on a tentacle to fill up its Stagger meter. Once full, it'll land on the ground and glow orange. Unload on it as much as possible to do damage to it now, and once its health is drained, press when prompted to chop off the tentacle. Repeat this process for the other three tentacles.
Once all four tentacles have been severed, you will have to deal with a nonstop huge swarm of regular enemies. They are infinite, so only kill them if you need health from them. The way to progress the fight is to keep an eye on edge of the arena nearest the monster, right in the middle. The monster will drop an object there, and if you're looking in its direction you'll get a prompt. Do so and Drax will pick it up and hurl it at the monster. You need to do this a total of three times (while dodging, fighting, and avoiding enemies all the while). After the third hit, the monster will come down and open up, at which point you'll get another prompt for Gamora to finish the fight.