That would be a good idea. It would be best to plan this as a defensive battle and plan to use our shock cavalry sparingly, largely using them to outmaneuver massed infantry. Particularly while engaged with our own infantry, they are too precious a commodity to waste sallying forth early (Unless we can cause a rout). In the early days we should largely focus on digging in and preparing for a siege. Our own archers and siege equipment will be of no use to us if they are countered in larger numbers by the offensive army. We should be building earthen works and creating shields to prolong the lives of our soldiers. Especially if we expose our archers by placing them high, we will want them to have some kind of shielding preferably something semi portable.

Pitch particularly fired from a catapult or the like would be a brilliant idea in the confined spaces of the canyon and is fantastic way to break morale of the offensive army. Especially when combined with caltrops to limit speed and maneuverability. 

If we can use some of Pizarks Hunters and trappers to find some goat paths leading through the mountains we will need to divert a small amount of men to protect them but it may also open up the ability to sabotage supply lines. 

But really there can be no concrete plan or clear way to deploy our forces until we know the disposition of the enemy. 

It is also important to remember we are not trying to kill as many as possible (Although more is better). We are trying to delay them.