Trial of Sekhemas Boss Ignores No damage Buff from merchant

Got to the end of the trial and had merchant right before the 4th and final boss. He sold me the no damage until the end of the next room. I tried it out when I got to the boss and didn't get hurt. That was until he stood in the middle of arena to spin and deal lightning damage. I wasn't trying to dodge and got melted. Apparently that move didn't get the memo.