It sounds to me like what you really need are some crossbows/arbalesters to take out some of those high-valour, armoured troops.

As antisocialmunky said, the ideal solution is to kill the general; however, assuming that doesn't happen quickly (which it probably won't) you can expect to suffer significant losses. I would advise against frittering away your light cavalry just to harass; save them, along with your knights, for a more decisive moment (as you said, charge in the flanks or rear).

Something I've tried occasionally is to stagger swordsmen/spearmen along the line, i.e. a unit of spears in the center, flanked on both sides by swords, with more spears on the ends of the line, refusing both flanks. This protects your flanks from cavalry charges and lets your swords engage the enemy line up front, so that your spears aren't slaughtered by enemy swordsmen and militia. You just have to be aware of your enemy potentially aiming their cavalry at your swordsmen; it helps to have some spears in reserve just behind your swords to guard against this possibility.

If you ambush with anything, it should be cavalry, which can quickly charge out of trees and bear down on the enemy from behind. I would use the gallowglasses as a reserve unit; they can deliver a potent charge to an already-engaged enemy and possibly cause a route. Woodsmen can be used in a similar fashion but won't do as well.

Make sure to use your armour-piercing longbowmen to target a high-value enemy unit, such as the general or some Varangian Guards. The archers would be better used on lightly armoured enemies.

Good luck; let us know how it turns out.