I’d recommend taking it easy firstly…
It’s very tempting to ‘Rush in’ on all enemies, but try to give yourselves some distance, hide behind structures, pick them off one at a time, try to seperate them, this can save unnessesary ‘extra lives’ being lost…
Try selecting ranged characters, or ones that have mass ground damage abilities in their skill tree… For example, Thorn: Bow (though slow, high damage), and her specials are ground based placements that can slow and damage multiple targets from a distance… Orendi might be a good choice for a similar reason (ranged attack, ground placement damage)… Both these characters are available straight away to use…
Collect shards, and buy ‘bots’ where/whenever possible, whether a healing one to help keep you alive, or an assault one to help also keep you alive but mainly by picking off any low enemies that might swarm you at times…
Move around, ALOT, it’s quite tempting to just stay put sometimes, but it’s okay to run for your life too, find a new position, give yourself space, draw an enemy a different way…
And everyone else pretty much covered the bosses, so I’ll just stick with my general ‘maybe these might help’ tips above, good luck!