This may help someone. I have two accounts and on both have been able so far to beat the opponents in the first try. On my main account I have a champion so that helps.
Bronzetalon (Champion) level 10 has beaten on first try 9 of the first 11 opponents in the list, while Mech level 9 was used to beat Aquarius and Taurus (4 & 6).
On the second account, with no champion, this is the list of dragons I have used to beat the tournament dragons.
Opponent - My Dragon - Number of tries.
1) Underworld 1 - Reindeer 8 - 1
2) Avatar 1 - Mistmoth 10 - 1
3) Medusa 1 - Honeybee 10 - 1
4) Aquarius 2 - Metal 10 - 1
5) Tiger's Eye 2 - Hypnotic 9 - 1
6) Taurus 3 - Metal 10 - 1
7) Melody 4 - Mistmoth 10 - 2
8) Acrobat 5 - Leopard 7 - 1
9) Firemane 6 - Mindvolt 10 - 1
I will update this list as it grows. Good luck!