I haven't played those battles in ages but,

just in case you're not aware of it,

be sure to have your spears set to H and F (hold position and formation), as well as your archers. the spears need it to get their bonus for being in ranks (they should be at least 4 deep, like Somebody said). the archers need it in order to not keep moving around every time they target somebody or somebody comes near. your spears should just sit there like a wall until your other units can flank. if/when the enemy against them becomes greatly thinned or highly surrounded, then turn the spears to E and let them engage at will.

target your bows manually, onto the worst of the oncoming attackers. once the attackers engage your spear wall, consider targetting somebody else who might be a better target - your spear wall will be blocking some of your archers, making them less effective in hitting anybody directly engaging the spears. but I wouldn't try to shoot enemy archers too much, esp. if they spread out... archers only hurt you in the long run.

let us know how it's coming. if you're still having trouble, maybe you could describe where it's falling apart.