Some extra details about your team would be nice but I can give general advice that will beat this map assuming you have some relatively optimal sets in your team. BL is a really strong house so it's pretty hard to screw them up.

IMO, just send single units out to block the reinforcements. If you cast Retribution on Dimitri, his battalion combo can completely annihilate the whole mage group so you can just solo him towards there and take out the mage that causes reinforcements. Going solo actually helps so they won't target other units. If you don't have his battalion combo set up, a dodge tank Ingrid will also do more than fine, but again, Retribution is key.

The far right one is best done by a flyer, I usually send Byleth with a Rapier down there and she has a good time. But anyone is fine, for example, if you have a Wyvern with Lancebreaker and Alert Stance+, then they're not going to have too many issues as long as you bring some concoctions and stuff and dismount when in ballista range.

Apart from that, it's honestly more than fine to just sit in the middle and delete the tyraid of enemies that come towards you. Considering how late you are in the game, you should have all your Swift Strike, Hunter's Volley and punchy boys all finalised so player phase should never be an issue as you can just one shot everything, and if it gets intense, AoE batallions are your friend. The main thing is just dealing with enemy phase, which is always just going to be about positioning, use of dodge tanks, support gambits if you need them etc.