In fourth edition Dungeons & Dragons, enemy spellcasters brought the same mix of encounter and recharging powers as every other monster of the same level. They posed the same threat, but with spell-flavored powers.
In fifth-edition Dungeons & Dragons, spellcasters have spells powerful enough to turn an combat into a quick victory. To balance this offensive power, PCs must make limited spell slots last through an adventuring day. But enemy spellcasters suffer no reason to pace themselves. As long as they remain in the fight, they can blast away.
In fifth edition, I have pitted a lot of evil spellcasters against groups of players, and I often see the PCs fail to give special attention to the casters. Big mistake.
In combat, in any edition, your party should focus attacks on one enemy at a time. By concentrating damage and eliminating enemies, you reduce the number of counterattacks your foes can mount. The fourth-edition Player’s Strategy Guide included a figure that showed the benefits of this tactic.
All this still holds, but the target with the most offensive power deserves special attention. That’s the guy raising a staff and speaking eldrich oaths. Find a way past the goons and hit the wizard with everything you have. Take him out fast. And stay out Lighting Bolt formation.
Protect the wizard
Smart monsters may use the same tactics against the party, so parties must work to protect their vulnerable wizards from attack. Is I explained in “Revisiting three corners of the new D&D rules,” the new rules for opportunity attacks rules can enable monsters to circle your front line and strike at the wizard in the middle. When possible, your wizard should stand three squares behind your melee characters.
If you play a wizard, then you must begin your adventuring day by casting Mage Armor on yourself. The spell lasts 8 hours and does not require concentration. Other defensive spells that do not require concentration include Mirror Image and Fire Shield.