Not counting the ton of nameds that always see invis on live, the generic frogs/minos/etc can also just randomly spawn with the ability to see invis.
And ya, unike PC wizards, NPC wizards hit as hard as melee mobs their level and have similar hit points to melee mobs. They hit hard in melee, have a lot of life, they have nukes that do a lot of damage, nukes that do a lot of damage AND automatically stun you and thus interrupt spell cast and of course pure stuns to just make you stand there and drool.
Of course there's lots of RNG that can go on. Like having a wizard root you and then the next spell they cast is a dispel that removes the root.
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