It's important, particularly if you're prepping for hard mode, to get characters that can either heal you or protect you from taking damage. Plant soul is fantastic for getting through, because of the constant passive healing. Dark horse helps with protection.
If you can get Medusa's legendary skill, that's a big help. It will allow you to store up more than full energy, to be used when things get harder. First few fights of the expo should be winning with autoattack, mostly. Just get energy. Then in the later rounds, is when you use it up.
Note, Medusa becomes almost essential when you start running into legendary bone drakes.
For quite a while, my standard lineup was plant, Medusa, ambush spider, Bardbarian, then whatever. If you use Bard's white while spider is in the air, he'll start swinging right away, and usually have killed something and be back at full energy by the time spider lands.
Now my expo lineup has changed with having legendary for dark horse and brozerker, and I can pretty well faceroll through insane difficulty.