-Play as Cable for this strategy. During Abyss' first form, block his attacks and repeatedly press [Shoot] until he dies. For his second form, keep your distance and repeatedly shoot until he dies. For his third form, use Cable's Hyper Viper to deal 90-95% of his life in one blast (a 115-130 hit combo). Then, just hit him once or twice with any attack to defeat him.
-Play as Megaman, Cable, and Cyclops. Use Megaman and jump behind him while shooting. During his second form, use Megaman and repeatedly shoot him. You should have a level three power bar by now. On Abyss' third form, wait for him to dive down before attacking. When he reappears, do a Triple Hyper combo. If the game is set on very easy and high damage, Abyss should die in one hit.