Rules Question - when to use mounts stats

It depends on the Troop Type. In this particular case, a Lord mounted on a Great Stag (which is a Monstrous Beast) has the Troop Type Monstrous Cavalry.

BRB p. 83 (updated): All the cavalry rules apply to monstrous cavalry, with two exceptions – monstrous cavalry always use the highest Toughness and Wounds characteristics the model has, rather than automatically using the rider's — indeed, this will normally mean that the model uses the mount's Wounds characteristic.

(The text in Italics is from the erratum.)

If the Lord is mounted on an elven steed, then his Troop Type is Cavalry, and "the mount's Wounds and Toughness are never used" (BRB p. 82).

To add: in both cases (Cavalry and Monstrous cavalry), the "rider and mount use their own Weapon Skill, Strength, Initiative and Attacks characteristics when they attack. Each can attack any opponent that the cavalry model is in base contact with" (ibidem).  However,  "it is the rider's Weapon Skill that is used for the purposes of the enemy rolling to hit — the mount's Weapon Skill is used only when the mount strikes blows. [...]the mount's Leadership is never used, unless a special rule states otherwise."

So on a great stag monsterous cavalry I would use the stags tougness and then the riders armour save and then the riders ward save?

note that the riders armor save goes up by a pip or two for being mounted just like for regular cavalry.

Here's a quick question: Can you direct Stomp attacks to characters in a front rank?

Okay, that's what I thought. I just watched a BatRep where a Goblin player directed one Troll Stomp toward a Runesmith, killing him with it, and the rest toward the unit. I didn't think that was correct.

Sorry to hijack the thread. It was just a quick question. Please resume.