$game_actors[id].tp -= 1# reduce actors TP by 1
I think you missed a .tp on thatI just modified my "in map slip damage" to meet your criteria
http://pastebin.com/icjM5KUf
Just write <slip tp: n> in state notebox
n is a number constant that will be used to reduce actor's TP.
(Btw, it's not tested yet)
anyway, if you want to manipulate actor's TP using event, it can be done by script call
$game_actors[id] -= 1# reduce actors TP by 1