Okay so you have a number of options:

1) Commit to battle to remove their firing options.
2) Use your missile troops to target their missile troops and thin them out before you run out of ammo.
3) Use your cavalry to force their missile troops to skirmish and/or commit to melee.
4) Soak up the damage with a single cheap unit in front of your own.
5) Stand around and take the damage.

Congratulations on choosing the worst option.