There is curious Arena event in location "Castle Ironfist" (not to be confused with standalone "The Arena" location).

This event can be triggered by going through the western teleporter on the hill with Castle Ironfist. You end up in area with obelisk (the event won't be triggered if you enter that area by foot).

Once you enter and each 30 minutes of in-game time a wave of enemies will spawn in that area. There will be 8 waves in total. Each wave consists of:
1. Follower of Baa x2.
2. Lizard Man x2.
3. Mystic of Baa x2.
4. Lizard Archer x2.
Total Exp Yield: 4624 Exp (1120 Exp for party of 4).

Curious thing is that once you enter Arena and trigger the immediate enemy spawn, then reload, enemies won't spawn upon entering, you will have to wait 30 in-game minutes. If you quit the game and re-launch it, still you will have to wait. It is like game tracks this event through Windows registry or something, I have no other explanation.

If you save between the waves it will also restart the time counter: you will have to wait 30 in-game minutes after reload. This might be useful to heal up your party if you participate in this event while being low level.

Also funny minor bug: if you check Auto-Notes, it will list fountains at Castle Ironfist in wrong order, it will say that east fountain adds to Speed and west fountain adds to Accuracy, but it is actually the other way around.