I just recently beat him, although Mario got KO'd at the last turn. Also Mario, Rabbid Luigi, and the third character.
I'd say, get rid of Rabbid Luigi. The boss is impervious to vamp/burn and cannot be tackled due to its size, so Rabbid Luigi can't play to his strengths (outside his cover-piercing bazooka). Rabbid Peach may be a better fit, she can heal, tank, and launch sentries to keep the boss busy (i.e. waste his overwatch skill) and still do a bit of damage. At least that's what I'd imagine.
What nearly did me in were the two (enemy type)
that spawn near the end. Get rid of them as soon as they enter the field, even if you have to take a risk. Preferably you'll have offed the two other adds before they become active, but since those are slow and lack ranged attacks you might be able to just run circles around them.
Other than that... Uh... Maybe beat some challenges for more skill orbs?