You have to earn dragon coins in order to unlock opponents. Each opponent you go higher will be difficult.

Opponent	How to unlock
Archos	Requires 2 dragon coins.
Boaris	Requires 30 dragon coins.
Kita	Requires 75 dragon coins.
Monki	Requires 100 dragon coins.
Mox	Requires 10 dragon coins.
Qu-Xian	Requires 200 dragon coins.