1x Omni mic anywhere under the ring on a short stand to give the low freq thud. (floor of the ring most times are mounted on springs to reduce shock)
2x Lav mics omni (cabled) to each of the contestants corners mounted along the rope (safely) a bit so they can sit in the corner and still have an exposed mic near the shoulder so you can hear them talking.
2x extra lav mics in the other corners (if you have enough mics / channels on the desk)
Fx mic on each backpack camera near the ring (normally 2 cams) Remember the fight will often end up outside of the ring.
Radio Lav mic on the referee... and a spare / quick change over of matches.
Direct split of the announcement mic or a split from PA.
Feed o/p from PA desk to get the intro music etc.
FX mic on the bell
There you go for what I would regard as minimum, .. Then add commentary setup and TV only mics for the opening of the program etc.
Foam pop filters on every mic to stop sweat that will be flying around.
Run an 8 way loon to under the stage and do the 4x corner lav mics and ring FX from that.
Should be an easy setup.... this sort of OB is always good fun.