Zombie mage's hit roll: 2 + 4/2 = 4
Trey's dodge roll: 11 + 9/2 = 15.5

So, he dodged. 

Trey's hit roll: 18 + 12/2 = 24
Zombie mage's dodge roll: 10
Trey's damage roll: (2 + 18 - 4)/2 = 8
Zombie mage's remaining HP: -12/0
Trey's remaining MP: 6/8