I think that if you hit 11 the positive effect on your party should be negative effect on the enemy Just opposite of what the party gets. Examples:

Corsair's Roll -Increases the amount of experience points earned by party members within area of effect.
11 Bonus -Decreases the amount of experience points earned by defeated party members lowering the enemies level.

Ninja Roll -Enhances evasion for party members within area of effect.
11 Bonus -Lowers the evasion of enemies within area of effect.

Just a thought...