Air Barrier

Create a Cube-like barrier out of air which can block projectiles, melee attacks and persons from entering. Should an enemy try to break it they would have to roll 2d8 and get higher than a 12 to break it, if they do not their attack will be repelled back towards them. dealing half the damage they would have inflicted. 

At higher levels the spell would scale by each spell level increased, you would be able to create larger and more durable barriers, the enemy would have to roll a 13 at the next level and then 14 at the next. Capping at 16