So, the spell says to discard monsters with total toughness equal or less than test result... If I'm fighting a Dhole (5 toughness) with 3 wounds, do I need 5 successes or 2?
It helps to think of toughness and damage as two different things.
Toughness is the monster's maximum health rating. This number will never change unless there is an effect that does so(for example a spell that lowers the monster's toughness rating by 2).
Damage is how far the monster's health has dropped from its maximum health rating (toughness) and is represented by the number of damage markers that have been placed on the monster token after a combat round.