[MaxWilson]

Common means for casting higher-level spells include:

Boosters. Skull staff (+1 D), Earth Boots (+1 E), Flaming Skull (?) (+1 F), Water Bracelet (+1 W), Robe of the Sea (+1 W).

Spell boosters. Summon Earthpower (+1 E, 4 Reinvigoration), Phoenix Power (+1 F, 50% Fire resist), Staff of Storms + Storm Power (+1 A). Underwater, Summon Water Power (+1 W).

Gems, as you mentioned.

Communions. Pythium for sure, and I've done it with EA C'tis.

C'tis doesn't actually need many boosters to cast some devastating combat spells. Cloud of Death has a huge area of effect and is armor-negating; its biggest drawbacks are the high research level and path cost and the low precision (-3). EA C'tis can get around the precision problem with the Nature spell Eagle Eyes, and 25% of Sauromancers can cast it without any boosters at all. Storm of Thorns isn't bad, although it's more a support spell for the entangling effect than an army-killer like Cloud of Death. Don't overlook the battlefield-wide spells like Rigor Mortis. C'tis doesn't have any undead national summons that I can think of, but you can still use Rigor Mortis with regular troops as long as somebody is spamming Raise Dead/Skeletons at the same time. Or you can get a bunch of Pale Riders (pretty cheap for C'tis) and a Wraith Lord. It's a great spell for making sure that a defeated enemy army gets dead instead of away.

-Max