[thejeff]

Slaves get the communion boost only for reducing fatigue from spells cast by the masters.

I think the guide is wrong about Soul Slay, though the rest of the description looks right. The confusion may have come in because you can boost some paths by 2, (PotS, & Summon Earth|Phoenix Power|etc.) And as you suggest, Banner (or Light) of the Northern star will get the slaves up to 3 S, though not 3H.

This is why Mystics make such good reverse communions, they tend to have the paths with easy booster spells. E1 or F1 jumps up to E3, F3 and they all get reinvig from Summon Earth.