Read this paperback novel about 10 years ago as a kid.
The premise is that one extremely powerful alien race has been harvesting (and presumably keeping in cryo-stasis or something) the best "champions" of other alien races in-order to have them compete in a fight to the death. The contest is held on Earth in a skyscraper or something, with the protagonist being an unwitting intellectual-type who was mistakenly trapped into the building with the other contestants. The human race's champion was a Roman Centurion I think. One of the other alien champions was a scorpion-like creature with stealth abilities, another has psychic abilities, etc.
If I had to guess when it was published I'd say 80s or 90s.