I like the idea of playing with AC. Let’s see if I can keep that going in 5e
Crack the Shell
You can spend 1 ki point after you make an strike that hits to attempt to sunder the foes armor. The attacked creature must make a Constitution saving throw or have its AC reduced by 2. A creature may only be under one of this effect at a time. The creature can have its armor repaired normally, through the use of a mending spell, or in the case of natural armor have a healing spell cast upon it.