Gollum in BFME. Although he's technically an evil hero, he's all but useless on the battlefield, with very low hit points, power, etc. It can be amusing to watch him attack enemies, as it always ends with him dying.
With the funniest scenario being Gollum vs Rohan Peasants. Watch as Gollum gets his skinny ass kicked by untrained farmers hitting him with everyday tools while Gollum does next to no damage to them.
Three words: Sauron vs Hobbits.
In a related note, players can come up with all sorts of interesting scenarios using BFMEII's map editor, including having three Saurons taking on hundreds of hobbits at once. Much carnage ensues.
Tom Bombadil in the second game, proving why you should Beware the Silly Ones. Summon him, and watch him skip, dance, and sing his way through wave after wave of enemies.