Here's how this went down...
Originally, I was going to have R'lyeh cast cure, but then I recalled that he'd be out of magic by now, so, instead, I had him try to light a torch...

Tactical roll: 3

So, he just sort of sits down and does nothing that turn.

I was also originally going to cast Cure on the zombie, however...
My hit roll: 3
Zombie's dodge roll: 16
So, I had my character opt to cure Trey, instead.
Healing roll: 6
I heal for 9+6 = 15HP... he's only got 7 HP total, so, I'm just going to assume that this will heal him back to full health.

Backstabs' to-hit: 20!
She does... 6*2 = 12 + 4 = 16 damage to the enemy (crits can ignore resistances, but not immunities)
the enemy is now at -6HP.