The trick is since Assurance ignores penalties, if you know it'll succeed then it'll always succeed, even if you're at full MAP. You can also bump a higher level enemy down into the "success" zone by inflicting debuffs to their fort or reflex DCs. A fantastic combo is Intimidating Strike -> Quickened Strike -> Trip (Assurance) on a fighter, setting up your teammates and an AoO with fair reliability. It's better to rely on your teammates to inflict greater status debuffs than Frightened 1 though.