I go into a Berserker Rage...So, despite being prone and grabbed, the walking corpse is also prone and should grant CA, canceling out my -2 to attack and allowing me to get sneak attack damage in.
To that end, I'll use Riposte Strike to attack Walking Corpse 4:
Attack: 1d20(15)+9=24
Damage: 1d6(6)+2d6(4,4)+6=20 damage and if he hits me or an adjacent ally (Vigilante Justice Style), I can make a Strength-based attack in retaliation.
By the way, if an attack of mine makes my foe bloodied, I'll use Press the Advantage as a free action to make another attack:Attack: 1d20(12)+9=21
Damage: 2d6(1,3)+6=10 damageAnd I'll ACTION POINT!
Funneling Flurry vs. Walking Corpse 4:
Attack: 1d20(7)+8=15
Damage: 1d6(4)+5=9 damage, and I slide the enemy 1 square (to
M8 if grab doesn't disallow forced movement)
Hit or miss, I'll mark the enemy.
If I am allowed to slide it with Funneling Flurry, I'll stand up. If not, no big deal.
I'll also use Sneak in the Attack as a minor action;
Until the start of my next turn, any ally who has CA against the enemy and hits it deals 2d6 bonus damage to it.