The girls are best used with a tough unit + magic shield + target. When you hit a lvl 3 on the target spell, any unit lvl 1-4 will attack it AS LONG AS IT IS IN ITS RANGE - meaning as long as with its move points. If it's outside them, it will attack the first closest target, and if that's the fairies, that's a problem. Ranged units will always attack the unit with target spell on it, AS LONG AS THERE'S NO ENEMY UNIT NEXT TO THEM (like your fairies who are taking them out).
With a little care and a little tactical planning, you can rock with the girl party. The only type of enemy you can't use them against is dragons. They'll simply eat up the fairies, so keep 2 strong units in reserve.
As for Mehgard, go there. A few enemies to kill for some more XP never hurts
