The boss mission Trigon's Blood,' a level 17 mission, takes place in a hospital. Once 
Zatanna opens the portal and you can 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 taking out all the enemies, you will notice several wheelchairs laying around. 
Destroying a wheelchair gives you 15 XP points. A few seconds after you destroy one, 
it respawns again, giving you endless access to XP. However, it is a slow process. 
This is one of the only missions where the objects respawn.