If I'm understanding you correctly, you should just use the movement trigger build into your boss trigger events, like "unit enters region(that circle of power is on)> move (your unit) instantly to (region/arena), And when the boss dies...if is an existing one make trigger "Specific unit event>(boss unit) dies> Wait 15 second> move (your unit) instantly to (region). If it's a spawned boss, it would be (generic unit even> unit dies> unit type of dying unit is equal to > (boss unit)> wait 15 second> move (your unit) instantly to (Region).
If this makes absolutely no sense, which I'm sure it doesnt, you can email me the map and I can trigger it for you. But I encourage you to try this first...experimenting with stuff is the best way to learn triggering.
-VGsatomi