We have 20,000 utterly expendable, impossible to break, fanatics of our own. Why don't we use them?
[X] Plan Mano a Mano
-[X] Have the Flagellants released from their collars in the direction of the portal as a wall of humanity whose physical mass keeps the nasty gribblies from getting to our army for however long they last. (Since presumably all the stuff Chaos Warrior and above would be hanging out around the portal).
-[X] for however long they last the cannons have free reign to ruin Papa Nurgel's day. Namely by blowing up the portal and the Chaos gribblies wading through Flagellant.
-[X]
this is an example of the inspiration for Ungol horse archers. (I don't even have to say anything about the Huzzars) As long as they have some distance or nearby Flagellant they can keep the Norscans busy until everything else has been killed. If things get too hot then they can slip in behind our lines with everything behind them completely incoherent.
-[X] if all else fails, the cannons miss, the ungols are overwhelmed, the flagellants are vaporized too early. Then the rest of our army, Knights, Ogres, and common swordsmen alike are to as turtles up as physically possible. Another wall, but of armor, pikes, and magic.
we're basically using all our strengths to delay, distract, and pin down Chaos while discouraging any bullshit cheating rituals by giving the Nugelites a long attritional battle at the end of the tunnel, and pissing off the Norscans by not dying.
All this leads to the Elves breaking out, throwing some magic while wearing fancy armor, and saving the day. Since that is basically the Elvish version of drinking beer.