You want really easy easy (albeit cheap) ways to get xp in Marvel: UA?
(the following does require Deadpool mind you, or anyone else with rapid fire attacks)
In Mephistos Realm, the point where you have to throw the demon things into the fire pit to lower the pedestal. Until you throw "x" number of demons in the pit they do continue to respawn endlessly. Place Deadpool (or whoever) near the edge of the cliff rite near the area the demons respawn. Aim at the respawn point and use your dual shot to waste them, you can hold the button and do this non-stop as they keep respawning and everytime you kill one or two your energy replenishes. I leveled up everyone like 5 times in about 10 minutes. Cheap but easy, plus you can get the 175 kills required for the costume unlock (yeah scarlet spider)