Here's a rough guide:
1) Do Shocker SQ (Story Quest)
2) Do Doc Ock SQ
3) Chapter 3, go to Safehouse and get your XP just for talking to somebody. Seriously, you'll level up just by clicking one button.
4) Go to Hand Tower
5) Go to Kingpin SQ and then to Magneto SQ
6) Talk to Xavier in his School Hub then off to Sinister.
7) Talk to Xavier again and then sadly, go to AIM Weapon Facility.
8) Kill all the bosses there.
9) You should be at the upper 40s at this point.
NOTE: This is the kicker and how this works. Go to Clea and buy a Neuralyzer IF YOU ARE NOT in the upper 40s. This allows you to reset story mode quests so you can DO THEM AGAIN for more XP again hahahaha. Cosmic prestiges love this as the xp is godly from 47 on up (This is when Doom unlocks and you start to do Heimdall and go up 4 bars if you're boosting hard).
10) If you didn't need a Neuro, proceed to Doom and Kurse. Once you're at 47, this is your new leveling rotation:
- Heimdall, Kurse, City Under Siege, Loki, Doom. Neuro if not at 60. Rinse and repeat. Again, this begins at lvl 47!