Wolves' Den arena is available at level 30, another one at level 40 and the last one 50.
Frontline - Borderland Ruins (2 modes) are only available to level 50 players.
More informations : http://ffxiv.consolegameswiki.com/wiki/PvP

BLM is op... well it depends. Are you talking about Adrenaline (personal limit break) in Borderland Ruins (Slaughter) ?