I have champions able to cast both wizard and cleric spells in my best deck, and have it well-stocked with 12 great spell cards. Let's take a look!
In Spellfire, you have to be able to stop events. It's even more crucial in the Antigonish variant of the game. My best deck has a Helm. It has a Delsenora. It has an Intercession, a Black Hole, and an Enter Darkness Together. It also has a Limited Wish. I know I'm going to face multiple nasty events every game. I need to stop Good Fortunes, Cataclysms, Calms, Slave Revolts, Curses of the Azure Bonds, Caravans, Black Besses, Caer Allisons, and who knows what else. Limited Wish is a big part of my event-stopping arsenal.
Even better than Cataclysm or Disintegrate, when you really need a realm gone for good, it's gotta be Estate Transference. Straight into the Abyss it goes, and it won't be coming back. Slight downside is that I can't attack my opponent's formation on the same turn I use Estate Transference, but this deck doesn't attack often, anyway. It's more a defense deck geared toward getting six unrazed realms quickly and efficiently.
"Counterspell" type cards are rare in Spellfire. I like being able to control what goes on at the table, so my best deck is well stocked with them. Dispel Magic is a tried-and-true way of keeping Wishes, Finger of Deaths, Creeping Dooms, and Mindkillers at bay while I build my pool and formation.
This is one of my favorite cards. After I raze my opponent's Ancient Kalidnay or Menzoberranzan, I still can't slap my own down, because of the Rule of the Cosmos. The land is razed, but it's still on the table, mocking me. Dissolution to the rescue! It clears out razed realms and allows my copy to be placed into my formation. It's also great for getting rid of Mithas or the Scarlet Brotherhood before they're rebuilt.
Sure, it's a bit overrated (mainly because of it's relative uselessness against avatars). But Mindkiller is still plenty good enough to make my best deck. The advantage it has is that it puts non-avatar champions into the Abyss rather than the discard pile, making it much harder for my opponent to get them back into play. This card is basically the cleric spell version of Takhisis's Abyssal Gateway. Except for the pesky "can't send an avatar to the Abyss" part.
This is one of my favorite Spellfire cards. It renders any champion - even an avatar - useless. It can't attack or defend, it can't cast spells. Gib Kcir can't be discarded to remove a card from play. Helm can't cancel events. This is one of the best "shutdown" cards in the game. I have three of these, and they're all in decks. Mindshatter is just that good.