I didn't mean 20% chance to render the mob unable to cast.

I meant, a tiger clawed mob will fail to cast 20% of its spells as a result of the injury to its throat.

I also was envisaging this as something that is capped at 20% (say). ie a deafened, clawed, mob with a dagger sticking out of its throat will still cast around 80% of its spells effectively.

So a group of three assassins would still face four spells out of every five on average. They couldn't just keep clawing until the mob couldn't cast at all.