Tua would be the most difficult task, because he had huge punching power and could deliver it after being outboxed. Actually, they both could KO each other. Tua had a granite chin, but still he was KOed even in amateurs and dropped by Rahman after the final bell. Povetkin would probably outbox Tua, hurting him on several occasions.
Brewster and Ruiz would be the easiest matchups. They weren't hard to be hit, Povetkin would methodically batter them both.
Maskaev could win on points several rounds, but eventually Povetkin would catch him. Maskaev lost a lot of important bouts by KO.