Say a full spellcaster is fighting an enemy that has cast an antimagic field on themselves, and due to either the enemy's high Con saves, the low damage output of their martial allies, or both, expects that the antimagic field is not going to go down any time soon.
What spells or types of spells can this caster usefully cast in a combat against an enemy with an antimagic field up?
Mobility via teleportation (Misty Step, Dimension Door, Teleport) or increased speed (Haste, Fly, Investiture of Wind) can keep them out of reach of the enemy, if it's strong in melee.
Some spells can make them or their allies harder to hit with ranged weapon attacks (Warding Wind, Investiture of Wind, Haste).
Others can provide total cover against ranged attacks (Wall of Stone, Wall of Force).
Healing spells or spells which remove/prevent debilitating conditions will keep their martial allies in the fight. (e.g. Heal, Greater Restoration, Dispel Evil and Good)
Shapechange or True Polymorph can allow the spellcaster to transform themself into a creature with strong non-magical ranged attacks, such as a Giant (rocks) or a Dragon (breath).
Situationally, Control Water could allow you to try to drown the enemy by causing water to flow outside the area of the spell.
Undead that the spellcaster has raised with spells such as Animate Dead or Create Undead will not become inanimate in the Antimagic Field - but the spellcaster's control over them will lapse. Given that the undead will at that point be right next to the enemy, that's probably not all that bad.
Prismatic Wall is explicitly immune to Antimagic Field, and can be shaped to contain such an enemy. Since they've got an Antimagic Field up, they must drop it before they can cast any teleportation spells to escape this trap.
The only spell which is capable of directly dispelling the Antimagic Field is Wish. Speaking as a DM, "Beat this specialised level 8 spell" is probably not the level of power at which I would start to bring in unintended consequences of the Wish. This does risk a 33% chance to never be able to cast Wish again, so it is a steep cost.
Control Water would allow a caster to flood the entire spatial volume immediately beyond the A-M field with water... any water inside the field is no longer "controlled", but since it can't flow away anywhere (because the water surrounding the field IS controlled by the spell) those inside the field are now in danger of drowning.
They can use an Artifact. From the Antimagic Field description, it doesn't suppress magical effects created by an artifact or a deity.
And since Artifacts are very special, they can do anything that the DM decided they should do. Such as cancelling any spell. Or it could cast Sleep or Power Word Stun, force everyone that looks at it to dance until they die, or anything at all that would interrupt concentration.
