Few Mercenary Questions

[victual]

It says I can only cast Asperio on my Merc if it is summoned by my priest.  If I am boxing him with my Sniper then I would have to alt tab everytime I fight something to get the Merc to join in?  Or would it know to attack if my Sniper was groupped with the priest?
Its looking like I will just have to summon it on the Sniper and forgo Asperio =(

Any places you've had great success mobbing to get kill counts?  Especially ones that can be FAS, Arrow Shower, Claymore'd with?