It is possible to get this achievement in the third level. You have to kill 30 enemies with the Wakfu-Gel (the blue orbs). Wait until an enemy is near a blue orb then choose your dragon and push LT.
If you wanna boost the achievement go to level 7:
"There's a constant supply of crabs at the beach while searching for the 5 clamp-sisters. Just attract 4 or 5 crabs. Then hover as dragon over the water (the crabs can't follow and will not attack) and kill on until a blue orb drops and push in your lifeforce (LT). As the crabs line up just right to be killed, you'll easily get the 30 kills here." Thanks to Quidac.