[Taqwus]

Enslave and Soul Slay could be nice, but it's possible to outfit an SC with an eye towards extreme MR (the most outrageous extreme kit might include Magebane, lead shield, Amon Hotep, armor of souls, antimagic amulet). And if the SC is backed by mages casting spells like Antimagic, ugh.



One might note that since communion boosts effective casting level, not only does penetration increase but also a number of spells increase in range, damage, number of effects or both. Orb Lightning, for instance, is an obvious one for Pythium to try against decently armored enemies. In terms of unresistable spells, Petrify is there but as noted is not easily available; Crystal Mages could help here (earth-2, astral-2; give earth boots for earth-3; cast SEP for earth-4 and reinvig). Against lightning-immune SCs with decent MR, you might want to put penetration-bonus items e.g. rune smasher (+2), spell focus (+2), even ring of sorcery or wizardry (each +1, and +1 to relevant paths) if you can.
Power of the Spheres + Starshine Skullcap + somebody holding Banner of the Northern Star + 8-15 communicants = +6 astral level; Ring of Sorcery, Wizardry, or a Tome of High Power would be another +1 without attracting horrors if you can afford it.
+6 astral should give on average +3 more penetration (+1 per 2 levels above what's necessary) +maybe +4 in penetration items (rune smasher/spell focus) should give ~+7 penetration on your Soul Slay/Enslave Mind which should make it not too unlikely to work against an SC who's meant to resist physical and elemental damage and not just magic.