Pretenders can cast high level spells, if built with high levels of magic. That is not the only way to cast those spells. There are several ways to do it.
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Site searching -sometimes you find hidden sites that give you different types of mages, like pyromancers, sorcerers, witches, enchantressses, etc. You should always be site searching. I find the “cast monthly ritual” combined with Augury, Hauruspex, Gnome Lore, etc. is the easiest and laziest way to go.
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Items - Construction of items that give bonuses to magic abilities is the most common. At low levels, you get earth boots, thistle maces, starshine skull caps, things that give a +1 to a path to the user. At artifact level (construction 8) you get items that give stronger buffs.
3 . Empowerment - take a commander, and click the empowerment options. For a hefty fee in gems, that commander now has a +1 magic skill.
- Conjure a better caster - in Conjuration, and somewhat in Blood and Enchantment, you can summon creatures that are perhaps better or different in magic than the mage who summoned it. Lama Queens, Ice Devils, Spectres, Liches, Forest Troll tribes, Hidden in Sand/Ice, and other spells all summon mages with various paths to help you branch out.
Its worth noting that you will probably use all 4 of these options in a game, if you want to win MP.