Can you only do it once or is it repeatable?
You can do it repeatedly but you get no real benefit after the first time other than Experience.
The first run through, you get eggs to get the pieces for the main prize and you have a chance to get the main guy if you’re quick enough
You work your way down through the floors to the bottom.
You will get a regular egg per level after the first, and a gold mission egg or the grand prize for the bottom.
You can leave and re-enter as often as you like, but you can only get the egg prizes once per floor per week.
And thrade gets the stuff I missed
I’m New so what level should my monsters be to get really far
Depends what you mean really far. If you want to have a chance at finishing it then your monsters should at least be in 80’s range. I remember when my top monster was level 25 and rest around 20 I tried doing OM for the first time and only got to like floor 3 lol. I was like “what the heck is the point of having minimum lvl 20 when you have no chance of finishing it anyway?” Despite training really hard I wasn’t able to raise them high enough before that mission ended, despite that I was lucky enough to win 1 Minoflare from the eggs.
So if all I said was TL;DR then all I’m gonna suggest is do it as far as you can because theres a chance you might win one from the OM eggs.