Armor of the Reversed

Touch one creature and cause a pool of a liquid like substance to surround thier body. If the creature is unwilling they must succeed on a Constitution saving throw with advantage to stop the spell from affecting them. The spell last 6 rounds

The armor causes any damage delt to be flipped. Take any damage to be delt to the creature and subract it from 20 (0 damage does not count). An example is if you deal 8 damage to the target of the spell at a 2nd level spell slot, the target will actually take  12 damage.

if you cast the spell at 4th level you subract the damage from 30, at a 6th level slot subtract the damage from 40, at 8th level subract the number from 50