I'd simply recommend just one roll like normal, and if it lands on a 19-20 it automatically crits. The skills that ups crit rate could be that 15-20 yeah.

What I meant by difficulty checks was something like the monster having a modifier against your accuracy as they grow stronger and all that. Like a monster with a 5 diff check would minus 5 against your natural rolls plus modifiers.

Just a suggestion. ^^;