No no no...
Liquid weapons are not allowed.
Explosives are not allowed.
Interfering with the oponent's frequencies isn't allowed.
Electrical discharge weapons are not allowed.
Missile weapons are not allowed either (so forget about an armour piercing shell!)
phreenet has it right: Robot Wars promotes a big show, as part of the competition: Battlebots doesn't. There's also Robotica here in the US, but I think they were short lived.
I like Airspirit's idea (counter rotating blades), but torque can be avoided in many other ways, more simply. Also, the torque can actually be beneficial, to keep the bot level.
That reminds me of the poor fellow who had a vertical blade that was so heavy, that the bot looked like it was about to tip over, anytime he turned left or right.
The nastiest one I've seen, is Son of Wyachi: three steel meat cleavers rotating at high speed.
The most clever, was DrillZilla, a bot with a fast moving no-wheels mechanism (which increases your weight limitation).
The most exotic, was a long snake-like thing, but I've never seen it in action.