My Solution

Ok, here are the characters and units I think would take this big bad boy out.

There are a few things that would work nice against it. Imagine the Warpscroll, but they are MR 1, so even if out of dice, he has a good chance to dispel it.
The Warp Fire thrower should eat this unit up and cause panic checks.

But to beat it in HTH, I submit the following:

Warlord with/ Warpstone Armor, shield, warpstone amulet, warpstone charm
Chieftan BSB w/Banner of the Horned Rat
30 Storm Vermin w/banner, musc. warbanner, shields

20 slaves w/musc.

6 Plague Censor bearers

Setup:
Turn 1: He is in the middle of the field, marching across. Characters are in the middle of the unit.

Turn 1b: I run my slaves in front of him and at an angle.
My clanrats move off center to the right.
The PCB's are well away from the action to the left.

Turn 2a:He charges the slaves who hold. With a Ld 10 I should make the fear test. This angles him badly opening his flank to my warlords unit. BSB is in the rear of the unit.
He slaughters lots of slaves, they autobreak. Doesn't matter if he pursues or not, he is so large I still have the flank.

Turn 2b: I charge. Since I cause fear, no test. I hit and max, so I have 5 across facing his 7 deep. On avg my general will kill just over 1 guy. My Storm Vermin kill 1/2 of one. His return attacks against my 3+ armor save do just over 1 wound.

He has static +4 (banner, warbanner, BSB, US) I have +7 (banner, warbanner, BSB, 3 ranks and flank). So I win combat.
Ld 9 re-rollable, he makes it.

Turn 3a: He jumps his general over there and challenges. I accept. His general will probably use the Great weapon. Its much better for him. So I swing first and do .33 wounds to him. He swings back and does .74 wounds to me, but also does .3 wounds to himself because of the warpstone armor.
Basically, he hits on 3+ wounds on 2+. I get a 5+ armor save (any saves rebound as a str 4 hit to him. 3+ to wound, 5+ ward) I then get a 4+ ward.
So he has done .74 to me, to .63 ot him.
Storm Vermin actually get to swing first, same Init, won combat last round. So .5 to 1, and I still win combat by at least 2. same as before.

Turn 3b: Same situation, except I'm looking for the time I fail both armor saves. Or 2 of 3. I have a 55% chance to save at least 1 if I have 2 armor saves. When this happens I use the charm to re-roll them. This will have the affect of lowering his wounds to me by .24 and raising my wounds to him by .30

I also move my PCBs up to charge him next round. Since his BSB is in the middle of the unit an cannot move, he is primed for the picking.

After round 2 of the challenge: Highborn has 1.56 wounds on him, Warlord has 1.24 wounds on him.
Yes I know you can't do fractions of wounds, but over several rounds, those odds will even out.

Turn 4a: Same as before: HB = 1.56 + .63 = 2.19 wounds, Warlord = 1.24 + .74 = 1.98 wounds.

Turn 4b: PCB's charge in, getting 3 on the BSB. So 7 EG + BSB in base.
Fog wounds = 2.68. 0.34 of those on the BSB. 1 wound to PCB's.
Attacks: 6 on BSB = 2.51 wounds + .34 = 2.85 wounds. Should be safe to kill him.
4 more on eternal guard = 1.675 + 2.34 from fog = 5.02 so 5 dead.
So EG = 12 attacks = 2.65 dead + 1 fog = 3.65 dead PCBs. For ease lets say 4.

Challenge: HB= 2.19 + .63 = 2.82 (not quiet dead), WL = 1.98 + .74 = 2.72 wounds.

He has stubborn 9, but no re-roll and he no longer causes fear.

Turn 5a: Challenge: I do .30 wounds to him and he dies.
Storm Vermin now do .67 wounds.
He still does just over 1 wound back.

Fog does 1.34 to Eternal guard. and 2 PCBs are wiped out.

So he does 3 wounds, I did 2 wounds.
But now I have +7 to his only +3. So I win by 3, he has Ld 6 break test.

Turn 5b: Warlord does 1.5 wounds + SV = .67 wounds = 2.17 wounds.
He does 1.1 wounds back. So now I win by 5 and he has a Ld 4 break test.

He will on odds fail that one and I break him.

His total points: 1320
Skaven points: Slaves 44, PCBs 102, Warlord: 178, BSB: 149, Storm Vermin: 310
Total = 783

Not quiet under half but still over 500 points less.

What do you think?