Fall back Team

All expect Hawk run forward until nowi can be hit by the cavalry archer. Hawk in that time goes to the river and waits there.(Otherwise the KI will act different)
Then you fall back. Only lyn stays to get a fight with the green guy. Don't forget to heal with Serra.
After that fall back again with all uniits until they stand at the starting area. Now you just use your weapon advantage and kill one after another.

Hawkeye

Lyn

Nowi

Serra