In this level, do NOT attack the summoning eyes (the ones in the pink square), as long as you are not instigating an attack on them, the pink eyeballs will continue to spawn.
It is probably best to do this on tadpole difficulty. It is far, far easier.
To be honest with you, the eyeballs spawn so quickly that I wouldn't be surprised if you got this achievement naturally. Only the very best players will not unlock this one through normal gameplay. However, if you want to 100% guarantee the achievement, just do not attack the summoning eyes.