I was in a toughman tournament a few months ago...
I won the first round match but then lost in the 2nd
to some kid who really knew what he was doing.
If you have access to a heavy bag, I would say that
working out on that is one of the best things you can do.
Also, find out what weight classes are going to be used in the
fight - for instance, my tournament had classes of 140-160 lbs,
161-185 lbs, and 186 + lbs. I usually weigh just over 160 but
was able to suck to about 156 and get into the lightest division
(where the competition, presumably, is not as physically strong)
dont drink for at least a week beforehand.
good luck