Head to Loures Battle Ring.
Select "Coliseum Arena" on the Temuair World map and enter the door on the right.
Note that there is no real death penalty on player vs player enabled maps.
You can simply speak to an attendant to ressurect and leave.
7 different Arena Bosses will spawn.
Kill them with curse, fas and light attacks.
Frost Arrow is highly recommended because their speed is ridiculous.
Sleep and Poison effects (including Cursed Tune, Wolf Fang Fist, Venom Attack) can cause them to fully heal.