Check youtube out for guides, there should be a few.Honestly I woild say just take your top 5 champs in and smash through it.
Honestly, just learn the basics of the game like how to dex, parry, intercept, learn the specials of all the champions, and you should be able to easily get through it. And if you are having trouble with act 5, act 6 is going to be a nightmare.
You've got enough to complete act 5 you don't need to rank up anyone I recommend using Hit monkey he will help you defeat 5.4.4 boss. Parry Modok and use MLM or MM combos when he's stunned. For the 5.4.5 Mephisto boss use kitty pryde because she's incinerate immune. Use hit monkey for the 5.4.6 boss. Use MLM combos and learn to dexterity Ultron's sp1. Read all nodes for each boss properly. You need to watch some tip videos on YouTube and learn the basics of fights. Learn how to use your best champs. There are guides on YouTube I recommend MCOC NOOB for story guides and Jason voorhees channel has 'how to use' champ videos. If you're still struggling put Heimdall in your team he has excellent synergy. It basically stops you from dying and allows you to safely counter attack