(For anybody else wondering where this is: this battle is near the end, where the paths split and you can choose to just run away or keep fighting the big bad guy.)
Are you having a problem with the initial boss-like spawn, "Eztli's Hand", that looks like a hand? Or are you having a problem with the "High Priest" that spawns after you defeat the hand, digits, and all the other waves (a little dialog appears when it spawns)?
I forget if the game mentions this, but you can click on a monster to see its stats, and you can hover over some of the icons, like the strengths (arm icon) to see its special strengths, and spawns (egg icon) to see what it spawns after it dies.
"Eztli's Hand" is only immune to blind, knock back, devour, freeze, stun, and psi shield. So most attacks should damage it. (I'm pretty sure I kept moving Markos down the line my first time through on each play-through, with maybe one or two others for support.)
The "High Priest" is immune to a lot of things (hover the arm icon to see the full list). So you just have to find something that is not in that list to damage it. Since it sounds like you've tried already and haven't found a good combination, just run the game at maximum speed to see what happens.
(Side note: you got everybody to level 60 in the first play-though? Eek, that would have been a bunch of grinding, even with the 300% XP option.)