To defeat Abyss in his second form, choose Cable. Keep shooting Abyss with the pistol and keep shooting till you have level 1 on the gauge. Execute Cable's Hyper Viper Combo. This should kill him -- if not keep repeating this pattern. The works if you are fast enough to kill him while he performs his short range attacks.

Choose Megaman, Cable, and Cyclops. Play under the easy or medium difficulty setting. At Abyss, use Megaman then jump behind him and shoot. During his second form, use Megaman and continue shooting at him. You should have a level three power bar by now. At Abyss' third form, wait. He always dives 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.