Barrier

Conjure a green sphere around a creature you can see, the barrier has 6d6 health (increases to 8d8, 10d10, 12d12, on levels 5, 10, and 15 respectively). 

While the barrier has more than 0 hit points, the targeted creature has a +1 (+2 at level 10) bonus to AC. If the damage reduces the barrier to 0 hit points, the creature takes the remaining damage and the spell ends.