i would first try to go with a holy freeze but because slow enemies are alot less dangerous to you and your merc. but if you arent leaching enough mana you might go for a might merc to boost your physical damage. i have never made a charged strike zon so i dont know if mana will be a problem or not because lightning fury takes a good amount of mana.
Think not disdainfully of death, but look on it with favor; for even death is one of the things that Nature wills.