So as I’m playing last night I came to the realization that Hero Points (HP) are finite.  Regardless of how much you play, once you collect the HP from any given level, that’s it sport’s fans- it’s time to bust out the wallet if you want more.
For this reason, I decided to scrap my current account and start over...and over...and over again until I manage to pull the only 4-star cover from the 150HP pile. Â (Wolverine...X-Force, I think?)
Here’s the fastest way to do it  (15-20min cycle time).
NOTES:
This works on Android.  I’m not sure if one is able to “Clear Data” from an App in iOS without completely uninstalling/reinstalling it.
Until you’re happy with your covers, avoid linking your App with your Facebook account.
Until you’re happy with your covers, don’t register a name for PvP. (Each time you reset the App it will lock out the previously registered name for use)
Okay, here we go.
In the H.A.M.M.E.R. map, complete the following levels in this order:
1st & 2nd tutorial battles. (mandatory)
1st & 2nd & 3rd& 4thNon-Dr Doom path (NDD) battles. (the path to the right of the tutorial battles)
After completing the 4th NDD battle for the first time, you will be awarded 50 HP and forced to enter the “Recruit Heroes” menu and recruit Hawkeye. (They updated the app so you’re no longer allowed to circumvent this requirement)
Replay the 4th NDD battle until you receive the other 50HP reward, then buy one of the 150HP covers.
If you LIKE the cover you got. Â Rejoice! Â Register a PvP name, link to Facebook, and enjoy the rest of your game.
If you DON’T LIKE the cover that you got, minimize and kill the app.  Then Clear the App’s Data.
Rinse / Repeat until you’re happy with your covers.
Enjoy!