If you include in game system feats, Cloud stomps. It's just his defensive abilities are too great for Deathstroke to overcome. Cloud is pretty much close to Hulk level power, much faster and better skills (minus hulk regeneration due to no magic).
@rpgr If you have evidence of Bahamut's attacks being equal to the potency of nuclear weaponry, provide it. He can still be KOed or incapacitated. Deathstroke has the feats and stats to suggest he can keep up with Cloud in a sword fight, also.