Koke the Crab is the boss of Sky Garden--the seventh adventure.
The minions are more passive in this boss fight and are easily managed if they don't group up too heavily. Still, I'd suggest clearing them out.
Koke is a big boss, making him slow but keep your distance from him and use the default gun as it is very effective. I wouldn't suggest melee weapons on the boss because touching the boss damages you and uses heavy attacks when you're near.
The boss' most devastating attack is a charged beam that he will pan toward you, so roll away and avoid it until it dissipates--this attack can oneshot you if you're not careful. He also has a giant stomp attack if you get too close.
The achievement pops after defeating the boss.