This achievement is given from beating the boss of the underwater or "Ocean" section of the game.
Theoretically, this could be passed with just the grabbing claw, but shield makes everything a lot easier and the weapons are needed a bit (but you can use the tracking needles to do the little shooting needed).
The danger here doesn't come from the boss itself but from the red needlepods nearby. If you get too near to four red needlepods, they launch a bunch tracking needles at you. You can simply outmanouver them to walls as always, but the structure of said space is tight and they come from all around you. There is a nice quiet spot in the water, near the boss.
Basic strategy is to break the three visible sacks in the bosses veins. Anything breaks them, even the explosive needles of the needlepods. When the sack are broken they can be stuffed with a blue amoeba, one for each. The water level rises with each stuffed vein, to the point of beating the boss.
Things to consider:
-You can get the amoebae at the end of the cave to the right. I found it easiest to hit the pod and make a run for it with the blue thing.
-The needlepods can be avoided and their shots dodged or the shield and weapons can nullify the needles. Only problem you might have is the needles popping the amoebae-thing.
-Tactic of "Drop and shield" works nicely. You don't need to take any damage during the whole fight.