Well, for HERO, I guess you would have to base the crit on either how successful the to hit roll was or on how many body a blow delivers.
by 0-1 no crit
by 2 A crit
by 3 B crit
by 4 C crit
by 5 D crit
by 6+ E crit
extra hits would be stun, bleeding would be body per full 12 segment turn, must parry would be a loss of 1/2 phase, stun/stun no parry a full phase as per normal stun rules in HERO
By Body done;
1 no crit
2-3 A crit
4-5 B crit
6-7 C crit
8-9 D crit
10+ E crit
lynn