The single target, MR dependant spells can be nice in some cases(horde of S3-4 mages spamming soul-slay on MA Ulm and his pretender quite early on) and can even be spammed to take out SCs.
As said, master enslave can destroy entire armies, and with enough boosters can let you steal an entire army - a high astral mage with a rune smasher, starshine skullcap, eye of the void and ring of sorcery will have(I think) +5 penetration and 2 extra levels of astral magic. If you have a higher level mage and add a reasonable communion to that, you could be looking at some crazy enslavement.
Example:
S8 base mage(don't know how, maybe pretender)
Rune smasher, starshine skullcap, eye of the void and ring of sorcery.
Add 16 communion slaves to that(pretty viable with many nations) and I think that is 4 more levels of astral.
That should be a level 14 astral mage in total.
That astral magic+his penetration boosters should leave him with +8 penetration - say goodbye to your army.
If you swap either of the misc items for a crystal matrix and use either pythian communicants or mages with slave matrixes you can even pull this off on the first turn of battle, although it will cost you 2 points of penetration(or 1 if you have a S9 base caster). But with the matrix idea, if you are the defender you can cast even before enemy buffs.
Also, please note that with the original set-up you have an enslave mind with +10 penetration - enough to take out alot of SCs/thugs(other than those with insanely high MR) within a few turns of casting.
An Oracle with S10 can also get that +8 penetration WITHOUT a communion - eye of the void/ring of sorcery/eye of the void/ring of sorcery gives +6 item penetration and +2 high level penetration.
So master enslave is very fun to cast
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edit: Said spell focus instead of ring of sorcery.