Level 10: Hawkeye
Level 9: Black Widow
Training for Level 9: Daredevil
Level 8: She-Hulk, Iron Fist, Nightcrawler
Level 7: Spider-Man, Luke Cage
Level 6: Kitty Pryde, Iron Man
Level 5: Captain America
Level 4: Invisible Girl, Doctor Strange.
And, man, I haven't found a better weapon in a couple weeks. I'm feeling way outclassed in PVP. I might have to *gasp* research.