Mercer had the tools to beat Tua. If he used his jab consistently, he could outbox him. I doubt he would though. I think he'd go to war with Tua, and Tua would outfight him. Assuming we're talking the 96-97 Tua. Mercer would beat the fatass Tua of the 00s.
Mercer is a weird one, he could lose to absolute dross one fight but then we all saw how he went toe to toe with Lewis and took it to the wire. An inspired Mercer wins imo.
Mercer looked like a good fighter but he very rarely actually won. Both at their bests, Tua by decision
