I've witnessed few upsets myself trying a tournament, such as an Ice Gremlin defeating an Ogre or a common Orc taking out the Ogre Champion.
The key to both of these victories is their movement and the trophies they pick up. The Ice Gremlin can run around the arena, collecting 4 trophies as opposed to 2 for the Ogre. Picking up

s and 2x

helped him to stay alive maximize damage, using a hit and run technique.
The Orc got lucky against the Ogre Champion as she danced around too, picking up

s and potions as well. Her first attack was blocked and she was crushed by the Ogre Champion but used a healing potion to stay in the fight. On her next attack she rolled

and added the 2x

trophy to the attack. The Ogre Champion failed to roll any

and was brought down to 1BP. The Orc was able to move away out of the Ogre Champion's range and run back for another attack, adding 1 more body points of damage, killing the Ogre Champion. Miracles can happen...