Good locations to level up:
Tower of Giants, Stage 3 - Usually 5 stamina per run, down to 3 on weekends. You can run this nonstop up to level 90 or so; ~300xp/sta (~500xp/sta on weekends) Do this on the weekend.
Weekend Dungeon (Master) - 25 stamina per run, drops 70-300k gold (if 2x coin event); ~400xp/sta
Hyperion Lava Flow, Stage 3 or Stage 5 - 10 or 12 stamina per run, also drops Titan skill up material and King metals; ~500xp/sta
Oceanus Falls, Stage 3 or Stage 5 - 10 or 12 stamina per run, also drops Siren skill up material and King metals; ~500xp/sta
Kronos Forest, Stage 3 or Stage 5 - 10 or 12 stamina per run, also King metals; ~500xp/sta
Starlight Sanctuary, Stage 5 - If you can take this on, why are you here? ~800xp/sta