Hmm...
against todays weapons, a BattleMech would not last very long.
But, Then again, it would still be able to do sizeable damage in a battle and, so long as it had some sort of Plated Reactive armour, it would turn the tide of any fire-fight so long as no anti-armour weapons come into play.
ALSO, on a unrelated note, just Crazy idea, but... what about Incendiary flechette-based shotgun rounds?
and what about micro-neutron grenades? neutron bombs can easily cook enemy's in a burst of high energy radiation, but the radiation has a very short life, dies away quickly, and doesn't physically damage its surrounding, useful for killing a target but leaving equipment intact.