For me, basically, I would use archers and horse archers to harass the enemy, luring them into close in on the Allies to prevent arrows from attacking (since if they attack, friendly fire would surely occur) so that they expose their own center. My heavy and super cavalry will then break their attack on his flank and charge straight at their exposed center, routing them, rendering their entire army leaderless. Well, you can't kill Alaric but you can rout him and force him to retreat or maybe kill his sub-commanders, right?