Marvel Avengers Battle for Earth

All the winning character achievements can be gained in versus mode. I'd advise you leave these selection of achievements until you've complete the campaign mode as 3 or 4 of them will unlock naturally from the battles you have to win. Also completing campaign will guarantee you have unlocked all characters.

Simply load through the menus, fight modes -> versus, select hawkeye and black widow and fight against your defenceless opponents.