FF + Mace PB will keep your Mercs alive the best at the cost of being rather one dimensional. With pets' lower base hit points and Mercs' poor resists (unlike demons), they don't have much to build off of, so you might as well aim to boost Def to avoid them taking damage in the first place.
FF does offer area denial as you mentioned. That can help the mostly ranged set. The further away the targets, the more likely the Mercs' cones will capture targets.
For attacks, a procced out Force Bolt can be useful not just for KB, but can add some damage, too.
If you don't go Mace, however, then I would recommend Cold (although it may be you have already used it and care not to repeat). You'd pick up defense of course, plus some resists, stealth, debuffs and some replacement soft control. And Infrigerate is also procable. Cold will keep you a bit more busy than FF if that helps in your gameplay Call them snow troopers for theme.