Soul Vortex is a kinda-sorta Summon Earthpower-like booster, in that I frequently use Summon Earthpower not to boost level per se but just to reduce fatigue from multiple casts of Strength of Giants, Legions of Steel, etc. In that sense, Soul Vortex + cheap guards (or *regenerating* guards) is a good booster that will keep you casting Shadow Blast, etc. for a long time.
The only thing is that the really good, high-end Death spells are low-fatigue anyway (Disintegrate, Cloud of Death), so in practice this is only really useful for spells from other paths when you've got a cross-path caster (or a thug that wants to buff without gaining fatigue).
Militia = batteries.
Use them in large groups or they'll rout.
-Max