OK this is tremendously ugly in Visio format but its a quick idea. It illustrates my rantings regarding the battle tactics (or lack thereof) in The Lion the Witch and the Wardrobe. First note how the defensive line is arrayed with the mountain cliffs on its flank. In the movie they arrayed themselves in front of the mountain, which had the effect of cutting off a clear withdrawal path, and also offered no protection of the flank. My plan protects the left flank. The enemy were essentially all mounted or unmounted shock troops, and they advanced in an unorderly rabble straight for the Narnian line. You array your heavy infantry at the front, preferably behind some kind of fortification such as wooden stakes (to ward off cavalry charges). The light infantry is directly behind them, and can rain down arrows as soon as the enemy is within range, which would be especially effective since the fortifications could slow down the advance. Then, your heavy cavalry pours out from the right flank, speeds around and attacks the enemy's rear and flank. At this point the heavy infantry and heavy cavalry would be fully engaged in melee combat, and you would want to command your light infantry to lay down their bows and prepare for melee combat. Use them as a reserve, rushing in to bolster weak or broken points in the line. You would also want to wait until the enemy heavy infantry had reached the line and was engaged with your heavy infantry before unleashing the cavalry's flank attack. By springing the trap too soon they could double back and overwhelm the cavalry, forcing a hasty retreat and possibly a rout of your horsemen. Once they were encumbered in combat and clambering over the fortifications, there would be no way for them to properly reform on their left flank and rear to drive off the cavalry flank attack.
Here is the diagram: