Except that that CR3 sorcerer had an effective AC of 21 until he ran out of spell slots for Shield, and then 17 after. >.>
Unless the writer that made that pre-module adventure was just dumb and didn't know how to balance properly.
It is very similar to a CR 3 Sorcerer... He had armor, had a strength score, and even had multiple attacks, used Sorcerer spells and Charisma checks. There are reasons for this, but if I had to translate it to a class he is some sort of weird Sorcerer - Fighter hybrid.
It was built from the ground up mind you. Unlike the ones in the monster manual which mostly follow class levels (with features stripped out for complexity sake). Looking at his stats he seems outright like a boss.
I still think the Shield feels like it is a little too good... A reaction to get +5 AC for the entire series of attacks a single opponent does to you is good enough... But I dare not change it because as written it is a great spell for normal wizards, not so much for wizards who are wielding heavy armor. This is where 3.5 would have stepped in...
Still this is the theme of the type of enemies this guy falls under. They are good at staying in the fight and even when you guys are level 10 they can throw you through a loop. (as said low CR isn't code of useless anymore). Oddly enough I don't think they have the most scary gimmick of this adventure path, since really it usually amounts to "I wear armor".
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Also yeah if people multiclass I am definitely going to apply that attribute bonuses appear at the exact same level they normally do regardless of what classes you take.