If the party allows that to happen, it deserves to die IMO. As most people state, combat typically only lasts a few rounds, 5-8 typically is a long combat. At one spell per turn, the Volo's Evoker will use maybe half a dozen of its 16 spell slots.
Nope. I have always used spellcasting foes with spell slots and the fights are challenging and exciting, not TPK (in general, if bad luck or bad strategy leads to a TPK--so be it).
Why are you looking at MotM? They changed the HD...
The Evoker in Volo's was CR 9 with 12 HD, not frickin 22 HD. Why they decided to nearly double the hit points from Volo's to MotM is crazy... 5E already suffers from hit point bloat, so I guess they figure why not just layer on some more.
BEFORE:
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AFTER:
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The Volo's Evoker worked well as a CR 9 foe. I've used it, and it was hardly a party killer.
Meanwhile, the MotM Evoker is a hot mess. It deals 4d10 + 3 damage, implying 17th level-type cantrip damage, which it can do
three times in a turn for way more damage than a PC could deal with similar attacks. And yet the spell save DC 15 implies a caster-equivalent level of 9-12 due to the +4 proficiency bonus.
Oi... make up your mind already, WotC.