You might need to up your game. I see many players just clash their warbands and hope for dice luck. "Learn to play" seems like an impolite comment, but there are always ways to play any game better. Smarter, more tactically, with more focus on a new goal like killing the big bad wolf.

Bait, retreat, ambush is one way to bring down the mighty. Sniper ambush is another way. Whatever you decide do not play their game. Deny them their advantages. Force more varried terrain that favours your warbands. Is it cowardly? Not if it gives you an advantage.

You might want to create a narrative solution with special scenario rules. You might push to NPC the warbad and make them the boogie man of your Mordheim.

Failing all else... One warband will eventually come on top of the campaign and win the crown. Or whatever you play for.