Like the use of smilies

Well swords and militia beat spears.

Also it sounds like he is using only twelve units. And no cavalry.

So you need to get 12 units of swordsmen and militia and another 4 units of cavalry.

First attack his spearmen with one unit each of swordsmen.

Attack his celtic warriors on one flank with 4 sword/militia and set them to hold formation.

On the other flank attack his celtic warriors with 8 sword/militia.

Once you have finished the celtic warriors march against the rear of the nearest enemy.

With your cavalry keep them far away from the battle until all dad's spearmen and celtic warriors are fighting then charge the welsh bandits. Hopefully they will run... once your swordsmen finish the celts they can help your cavalry.

May not work but it may give your dad a few more grey hairs