You roll a d20. Or anything that can kind of roll. Like animals, bags of chips, etc.
TEM rolls a 14. However, Toad gets defense against vegetables +1 for his potato peeler.
Locked in battle, TEM's staff is reduced to half its stature and Toad's potato peeler breaks, resulting in a sharp pain between TEM's ribs. +3 metal indigestion bonus!
Troll/homunculus rolled 2 against Rao and caused a scientific (not magical) portal to Dimension X to open in the ceiling.