Select Cable. For Abyss' first form, just block his attacks and then press Y to shoot. Repeat this until he dies. For his second form, distance yourself as far as possible and press Y to shoot. Repeat until he dies. For his third form, just use Cable's Hyper Viper. It takes off 90-95% of his life in one blast and will be a 115-130 hit combo. Then, just hit him once or twice with any attack to defeat him