Such a late reply, however I have previously heard that a Carnotaurus is best for duels. Yet, it depends on the way you use implants. Vitality Implants are mega important with duels and pretty much everything when it comes to Level 15+ animals to kill. Strength Implants can be particularly good for duels if you are hoping to be ultra strong and very hard to kill with the Strength and Vitality Implants bound together. Armor Implants also make you more difficult to kill. In my opinion, I'd go for:
Carnotaurus with:
Vitality Implant
Strength Implant
Armor Implant