One-Winged Angel. Massive durability and close range gun use. X and Wesker can last a while, but they'd do better in a larger map. Sephy also has superior magic because even though Vivi can't be targetted, AoE can probably still get him, liking slamming other people towards him or shotgun spray or massive Masamune slice. Plus, can't Sephiroth fly?
Bullets and melee take out anyone squishy in five seconds. Wesker/X have nowhere to run. Though... I'd like to see a Wesker vs. Hayabusa Dirna. Still. Luca owns, Leon's shotguns, Sephiroth spams from above.
Also, wow, when did Luca and Sephy wind up back on the same team? Worthless maggots.