I am in fact getting much closer to my desired NPC by adding many extra spell slots to the creature skin. The NPC is able to continue casting spells significantly longer than before. I thought, from a test I did a long time ago, that bonus spell slots didn't help NPCs, but apparently I was mistaken.
The issue was, my epic wizard NPC was running out of spells (particularly against groups of 3 or more players), and then resorting to melee. He's still very tough and takes out his fair share of PCs with spells, but I want more. I want him to be able to throw out 5 lightning bolts, 5 fireballs, 5 scintillating spheres, 5 mestil's acid breaths, 5 gusts of wind, and have room to re-cast 3rd level buff spells.
If I make a sorceror, they will cast more spells, but they tend to spam the same ones in whatever AI setup we use. I prefer the control and variety I get from wizard NPC casters.
My ideal NPC spell caster might regenerate their spell uses a random number of rounds after they cast it. This way, they cast a spell and cannot recast it for a while, forcing them to move onto other options. When prior spells become available again at random intervals, they could use them again. It prevents spamming, and the random regeneration of a spell use keeps the player guessing a bit at what will come next.
Some of our players have our NPCs down to an art. They swap out immunity items at certain points because they know exactly how many times an NPC can cast something, or at what point they generally do. They use summons to draw spellfire until they can reduce NPC casters down to weak spells and melee. I guess in the end, I want their lives to be harder
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