What you need in that chapter is two good tanks, and a good horse slayer (Chrom could do it, someone with a beastkiller, etc. just someone that can kill a lot.)
Place the two tanks on the two spaces of choke right in the beginning, but make sure that they can survive the paladin. Also kill the knights that are already in the choke, if you don't have a beastkiller you can get one from that knight.
The rest will be behind them, the healers healing, and the attackers killing the left unit group. After that it's just waiting for the groups to move, then killing them. Sorry I couldn't write it more clearly, im in a hurry.