What mods do you have installed? Just SCS? Planetars aren't immune to Stun/Slow/Blindness and Greater Malisson, and only have Heal memorized three times. You can send in someone with Death Ward+Stoneskin+Mirror Image+Celestial Fury after casting Greater Malisson to get a Stun off, use PW: Stun for a secured kill (they do have 75% MR, although you can easily deny it completely with 3 Lower Resistance after a Magic Resistance).
Other than that, have it focus the other mages, or use summons. The Staff of Fire can summon infinite Fire Elementals if you have a Project Image use it (I think Simulacrum does the same, not sure tho), although this is useless if you lost your stuff (same for the Celestial Fury tactic).
I have recently caught interest for the Totemic Druid. I have been playing him since yesterday starting in BGEE. I have been loving him so far. I have always been interested in a powerful spellcasting class that focus on summons. I have been reading the Tale of the Totemic Druid, and I really love it. A shame I didn't find this earlier. Lovely material for a tuesday reading!