You won’t get the Rage ability until the third level. If you wait until the end of level 8 and you can get both this and Butcher at once. You’ll need to have both your Evolution and Rage abilities near maxed out so you have more health and consume less energy.
At the very end of the level 8 (The Base) there will be specimen containers with Explorer aliens in them (four of them) and four gun turrets on the area you have to hit the switch in order to finish the level. When you hit the switch the level won’t end until you have killed all 10 or so enemies that come out. Don’t turn on any of the gun turrets, let the Spargs gather around you, locate the three Gunner aliens, and then head towards them, making sure the Spargs follow you. Once they’re all around you activate the Rage ability by pressing then and tear them to bits.