So, here's what ended up happening this round, roll-wise.

My hit roll: 20!
Zombie's dodge roll: 5 :P

'bout time that happened, right? the crit ends up dealing 24 damage. 12 from INT bonuses, and 12 from it being a d6 spell.

Backstabs' hit roll: 16
Backstabs' damage roll: 5
Zombie's dodge roll: 7
Backstabs' effect roll (d8): 1
She hits for 5 damage thanks to the zombie's 6 defense.