Can't miss final boss fight. Giant Lobster like monster. Find Bryllcream, let him throw you and aim for the boss's mouth, may take more than once as the monster spits fireballs at you. Once in the mouth, hit b repeatedly. You will be spit out. Then, you have to attack two tentacles while OD attack you. I found the TNT Teddy Bear to be the most helpful.
After you kill the tentacles, return to Bryllcream and get thrown back into the mouth of the monster. Once inside, press b repeatedly and he will spit you out again. After that you have to blow up tentacles again, use TNT Teddy Bear. Repeat the scenario with Bryllcream. Then you have to defeat four tentacles, each tentacle will move position after taking a certain amount of damage (but not too far). After you destroy the four tentacles return to Bryllcream to be fired at the monster for the last time. The achievement should pop once the monster is down.