Level gap balance is something I've talked about before. Ideally, the math should ignore level and give everyone an equal chance of hitting / resisting each other. But that's a mechanics change, not something a guide could do.
Some thoughts:
- Make everyone in the same bracket be the exact same level. Players may have to delevel or gain levels to compete.
- Delevel players before the event and pump them back up afterwards (more work for you, unsure if you can do this).
- Don't worry about it. If players don't level up / delevel ahead of time then they can deal with it.