Bubble barrier

A bubble of water with a 10ft radius surrounds you and follows your movements. Any other creatures attack that deals bludgeoning, piercing or slashing damage that passes through the bubble has a -2 to hit. If the bubble is hit exposed to cold damage the surface frosts over, increasing the attack debuff by 2 (i.e. -2 to hit becomes -4). If the bubble is exposed to fire damage the frost melts, negating its effect until reapplied. Additionally, those within the bubble have resistance to fire damage. 

At higher levels the primary to hit debuff equals the the spell slot level used.