If you have never played a roguelike, the objective is to find stairs to the next level until level 10 where you have to beat the boss.
Each level has a different set of challenges / enemies but the general strategy as in most roguelikes is to be cautious and not engage with enemies. Be sure to use the possession power when you health is low, or to avoid floor hazards. You can wait possession cool down while in safe areas and make sure to try the special power of monsters.
To beat the boss, the easiest strategy is to possess a wizard and summon mobs as meat shield. Then leave the boss's field of vision to not be caught in fire. There is a different ending depending on which monster you possess while beating the boss.
Big spoiler: your original body is sitting in the middle of the room, invisible unless next to it. So lure out the boss and possess it for a fun ride.