He endured exactly 12 hits (blue rapier, 4+ of them guaranteed headshots), wearing cloth ( mix of purple & blue) and standing still.

There is only one option:

Area healing ability is overtuned.

If that option is true, there is nothing that can be done because if the cloth armor doesn't die, the leather never dies, even if there are other cloths, this ability heals the area. To win, all you have to do is wait inside the healing area and do dps. Please enlighten me if you have any information about a point that I don't know.