The Questing Knight, Ogre style (Hunter time!)

Personally, (since frequent ward saves are not usually I come up against when i fight) I would just give your hunter the Armor of Destiny with a great weapon and be done with it.  He has a 5+/4+ and the stonehorn has a 4+/--. 

Think if he's against a hoard of Empire State Troops w/ Halbers.  Assume they have a warrior priest and he gives the unit hatred and they have a combat hero that declined a challenge.  7 in B2B and 3 deep for attacks and a champ = 21 attacks.  Warrior Priest has 2 attacks.  Both need 4s to hit and either 5s or 6s to wound.

Assume they pass Terror and Fear.  23 attacks, 11.5 hits + 5.75 hatred hits = 17.25 hits.

If they target the hunter: 5s to wound.  17.25 * 1/3 = 5.75 wounds.

Armor save 5+ reduced to 6+.  5.75 * 1/6 = 4.791 wounds.

Ward save 4+.  4.791 * 1/2 = 2.395  wounds unsaved.

If they target the Stonehorn.  17.25 * 1/6 = 2.875 wounds.

Armor save 4+ reduced to 5+.  2.875 * 1/3 = 1.9166 wounds unsaved

Both are close to 2, so assume they score 2 wounds.

Empire combat res is 2 wounds, 3 ranks, banner.  That means that you have to deal 7 wounds out with 3d3 +d6 S6 auto hits and 4 S7 attacks.  It shouldn't be that hard.  Also, this is assuming that the hunter does not have a T buff, the hero declined the challenge, and that the state troops do have hatred. (which is a pretty common situation)  The hunter and stonehorn would definitely win the first round, but they would definitely need assistance because the hunter would eventually get dropped after 2 or 3 rounds of fighting and the stonehorn would not do well all alone.  All in all, good idea, but it isn't going to win against core with any kind of balls in hoard formation.

Well, i've been trying to find ways to use the Mounted Hunter myself. But one thing i think i would never do, would be charge a unit straight in its front, the whole idea (imo) is to flank charge, "clip charge" or even charge at monsters that your opponent has. I think that most people i play friendly games with would even attempt to let their monster be charged by the stonehorn, by that i mean they would align the unit so that they would collide, i charge or he charges. For fun at least...
Although we do have ironblasters for monster hunting

I'm thinking the real importance of the OTS comes in when this model goes into a challenge.  Sure, some folks will refuse and hide in the back, but then you're no worse off than running a rare choice stonehorn (other than the extra points).  Like others have said, target selection is gonna be important, but that's most always the case with our army.  Not a whole lot you can mash into the front of a bunch of halberd nurgle warriors and come out on top with...

Also, I hate the ice phoenix, it has a ward save, and this will help kill it.  I'm sold!

I'm going to try GW, charmed shield, 5+ talisman, OTS, and see what kind of results I get.

The OTS also works great against Phoenix guard as well.  S4 attacks will struggle to wound you and forcing them to re-roll ward saves goes a long way to reducing the unit down to a manageable size

Also works great against daemons.

One last thing, don't forget that the hunter gets an extra AS due to being mounted, in the example given above with the Armour of Destiny the hunter will have a 4+ AS, 4+ ward.