Mage

The Mage has three answers a day, so make it spend them

How to run it. 81 Hit Points, AC 15, and a turn of three Arcane Bursts at +6 for 3d8 + 3 Force, reach 5 feet or range 120. That is about 27 damage a turn against AC 16. Twice a day it can cast Fireball as a level 4 spell (9d6, average 31.5, DC 14 Dexterity) and once a day Cone of Cold or Fly. Misty Step comes as a Bonus Action three times a day.

The reaction that matters. Protective Magic lets it cast Counterspell or Shield as a reaction, three times a day total. Shield adds 5 to its AC: a +7 attacker who hits AC 15 65% of the time hits AC 20 only 40%. Counterspell shuts down a caster's big spell. It cannot do both in one round, and each use burns one of three.

The choice that matters is which trigger to answer. A good Mage takes the sword hit and saves the reaction for the Hold Person. A mage that Shields against every attack has nothing left when the cleric casts.

The common mistake is standing still. It has Misty Step, a 120-foot attack, and 81 Hit Points on a body with Strength 9. Keep it at range behind cover, and teleport the moment someone reaches it.

For players. Bait the reaction. A cheap spell first, a big one after. Once three reactions are gone, it has none. When you pick a save, avoid Intelligence (+6 fails a DC 15 save only 40% of the time) and aim for Constitution (+0 fails 70%).

The human angle. Baiting a Counterspell is theory of mind: guessing what the other side believes and wants, then acting on that guess. Premack and Woodruff named the idea in 1978. A Mage fight is two people doing it at once, the DM guessing which spell matters and the players guessing which reaction is left.

Source: Premack, D., & Woodruff, G. (1978). Does the chimpanzee have a theory of mind? Behavioral and Brain Sciences, 1(4), 515–526. doi:10.1017/S0140525X00076512