Hello,
I'd highly say that you shouldn't be boxing a monsters on level 37 or take more than 2-3, why? Because it's just not worth it.
The attack rottation should be: weapon > exori ico > weapon > exori > weapon > exori hur .. over and over.. and meanwhile mana up when you need it, basically on higher lvl EK you will need it all the time. It's about the base damage dealt which exori ico > exori > exori hur
But answering the question about exori mas then i'd say that exori mas should be used instead of melee spell if 3 monsters more, let's make quick approximate math (obviously dmg. depends on skill) let's say you hit 100 using exori ico to 1 target, then you only need to make 1/3 of the damage by exori mas so as far as it deals more than 34 dmg each it's obviously more worth to use it instead.