Yeah its just for the unlikely event that you don't kill the devils and both devils use evil slash at the same time.Though if the point of having over 12k is just for the devils, then I might just take the risk and spend a stone if need be.
You would still need more then 11319 hp if you plan on stalling on MK.III.
If not you you can stall on the mask (estimated guess) target mystic mask and make a 16x atk with 6-7 combo should leave one mask to stall on. However you will have to kill MK.III before her strikes, he is on a 3 turn counter but he could be on a 2 turns to start the round you could be lucky and he is on 4.