All of the "Defeat 150 Enemies with XXX" achievements can be grinded (ground?) at the end of level 1.
To begin, refer to:
The trick to this is not going beyond step #1. Do not destroy the generator with Humungosaur. Enemies will infinitely respawn, and because this is level 1, they are all easy. Just keep racking up those kills.
Don't worry about dying. The kill count is cumulative, and does not reset if you die.
Once you get the achievement, move on to the next form and repeat.