Once you have at least Possession rank 1, find a huge swarm of rats (pretty common later on), possess one of them via , wait for him to die, possess another, and keep going until the achievement pops. Make sure you have a good amount of mana potions beforehand as it will require about 9 rats to achieve. The period in which you must possess is accumulative. You do not have to do it for 3 minutes straight (it is impossible anyways). This can be done on animal targets at level 1 and human targets at level 2.