You don't cast Power of the Spheres. You forge Crystal Shields for your combat mages.
It's still an edge case I don't think I've ever seen! But yeah, the oft forgotten Crystal Shields.... If only your encumbrance was lower (but then you'd be OP so...)
Truly E/S is best support from lab crosspath.
And because timing:
If we're talking about communions, and you want your slaves to benefit from the fatigue reduction from having higher levels in the spells the master cast, you can't rely on Crystal Shields since item spells get cast before the communion goes up or any slaves join (I'll firmly ignore Crystal Matrix and unit id shenanigans for the sake of this discussion)...
If you give EVERYONE a matrix..... It may work.
Maybe. Would be horrifically impractical though.
(On the fatigue: Really? Huh. Cracks show up every now and then I guess. It has been a while since I read the manual after all. But my question now is: Where the hell did I get the 15 from?)
GOD DARNIT I TYPE SLOW.
PotS is expensive to me because of fatigue, S1 mage knocks themself out casting it after all, and said S1 mage can't gem boost because it costs a gem. It's a decent communion spell though for mitigating slave fatigue and/or giving a master just a bit more power