[Deathjester]

What you did should work, and has worked many times for me.

Could it be that the casting order is wrong? The master needs to act after the slaves, as the slaves can not cast spells if the master has already done so that turn.

That's the only explanation I can come up with.

Also, see this for reference from I assume the same guide:

"Reverse communion - 2 or 3 masters then as many cheap slaves as you can muster. Your masters will cast power of the spheres then whatever other booster is appropriate (phoenix power, etc). For air spells you can use the third master to cast storm so that air power is an option. Now, all your slaves have been boosted 2 levels, so they can cast fun things like falling fire, thunderstrike, etc. Even lowly S1 mages are now capable of Soul Slaying. The fun thing about this one is it's a great way to use all those cheap researchers you've got with no other preparation. Leading a fight with three rounds of 10X Falling Fires makes quite a difference considering how cheap the mages are. Note, this communion takes advantage of one of the quirks of the communion, the fact that slaves can cast spells so long as they act sequentially before all masters in the turn sequence. Commanders resolve their orders in the same order that they're listed in the strategic screen. If for whatever reason you're not comfortable trying to get this order right you can have the masters retreat after casting the buffs - the buffs remain in effect. Note, the communion bonus does not affect slaves casting by themselves, only the buffs the masters cast help. Apparently the slaves are acting by themselves outside the communion."