Give me a 20th level Diviner Wizard, let's roll initiative, and surround them with a Fighter, a Monk, a Barbarian, and another Fighter for kicks. Unless all four roll above the Wizard, the Wizard will not only get out of danger on their turn, they will potentially avoid all damage leading up to their turn, and they will likely cripple or screw over two of the martials as they get away from the other two.
Four Diviner Wizards around any martial is likely dead at the end of the round. Several hold persons, several finger of deaths, several wishes, etc etc; martials don't have a quarter of the resources fullcasters do, especially at higher levels.
MONSTERS WITH C LASSES
You can use the rules i n chapter 3 of the Player's
Handbook to give class levels to a monster. For example
you can turn an ordinary werewolf into a werewolf with
four levels of the barbarian class (such a monster would
be expressed as "Werewolf, 4th-level barbarian").
Start with the monster's stat block. The monster gains
all the class features for every class level you add, with
the following exceptions:
• The monster doesn't gain the starting equipment of
the added class.
• For each class level you add, the monster gains one
Hit Die of its normal type (based on its size), ignoring
the class's Hit Die progression.
The monster's proficiency bonus is based on its
challenge rating, not its class levels.
Once you finish adding class levels to a monster,
feel free to tweak its ability scores as you see fit (for
example, raising the monster's Intelligence score so
that the monster is a more effective wizard), and make
whatever other adjustments are needed. You'll need
to recalculate its challenge rating as though you had
designed the monster from scratch.
Depending on the monster and the number of class
levels you add to it, its challenge rating might change
very little or increase dramatically. For example, a
werewolf that gains four barbarian levels is a much
greater threat than it was before. In contrast, the hit
points, spells, and other class features that an ancient
red dragon gains from five levels of wizard don't
increase its challenge rating.
You can use the rules i n chapter 3 of the Player's
Handbook to give class levels to a monster. For example
you can turn an ordinary werewolf into a werewolf with
four levels of the barbarian class (such a monster would
be expressed as "Werewolf, 4th-level barbarian").
Start with the monster's stat block. The monster gains
all the class features for every class level you add, with
the following exceptions:
• The monster doesn't gain the starting equipment of
the added class.
• For each class level you add, the monster gains one
Hit Die of its normal type (based on its size), ignoring
the class's Hit Die progression.
The monster's proficiency bonus is based on its
challenge rating, not its class levels.
Once you finish adding class levels to a monster,
feel free to tweak its ability scores as you see fit (for
example, raising the monster's Intelligence score so
that the monster is a more effective wizard), and make
whatever other adjustments are needed. You'll need
to recalculate its challenge rating as though you had
designed the monster from scratch.
Depending on the monster and the number of class
levels you add to it, its challenge rating might change
very little or increase dramatically. For example, a
werewolf that gains four barbarian levels is a much
greater threat than it was before. In contrast, the hit
points, spells, and other class features that an ancient
red dragon gains from five levels of wizard don't
increase its challenge rating.