This time on Warrior difficulty, I've gone for this party:
Human Mercenary (two handed/crossbow/medium armour)
Elven Bladedancer (daggers/duel wielding)
Orc Shaman (Earth/Water/Mace)
Dwarf Runepriest (Fire/Light)
Not so sure if the Mercenary is a good choice. It's quite a bad class design making a character "jack of all trades" and expert of no weapon type, as the grand master skill is particularily strong. The only advantage the Mercenary has is his Class Ability to always crit, IF he attacks enemies with full HP (this also means you shouldn't shoot at the target with ranged weapons). This is especially important because you can combine it with the sword ability which causes double attacks if you crit. You have an extremely strong first strike once you reach Sword Master. But only then. If you don't have the combo of Sword+Full HP then the mercenary basically has no advantage.
Imho they made a couple of big mistakes when creating the character systems:
1) The mentioned issue with the mercenary (no GM skill and being locked to Crit combinations)
2) The Ranged Weapons Skills are almost useless (as their contribution to combat is minimal). I recommend to equip everyone with a ranged weapon and not to skill it.
2b) Therefore ranged classes make no sense (luckily they also have a melee skill at Grand Master but the Class ability is wasted)
3) They added a Tank class which focuses on tank skills. But M&M X doesn't have a tanking system (using taunt all the time isn't feasable). Key to victory is to increase HP on all characters. Characters who are unconcious don't get any XP. And it makes no sense to have just one "sturdy" character who is always still standing while all the others are lying dead on the floor, yelling "Me Tank!"
4) As mentioned before, Double Attacks are completely out of balance and stronger than everything else (Dagger Blade Dancer with double attack weapons). This is especially true as every single hit triggers all procs.
There are only few games where you have such a "simple" character system where the developer fucked up so many times and allows the player to make so many mistakes (which might only reveal later on).