The boss mission Trigon's Blood,' a level 17 mission, takes place in a hospital. Once Zatanna opens the portal and you are able to go down the elevator shaft, you will find several rooms before actually coming to the Boss, Brother Blood. If you look around the rooms, after defeating all the enemies, you will notice there are several wheelchairs laying around. Destroying a wheelchair gives you 15 XP points. A few second after you destroy one, it respawns, giving you endless access to XP. However, it is a slow process. This is one of the only missions where the objects respawn.

Another strategy by Ali Momin: On gorilla island, stay after you complete the mission and go back to the first area where Tiny is. Here you'll find barrels on the top level that are worth 11-12 XP that instantly respawn.