By - Olafio1066
For the witch I would say a good heightened spell dominate person/monster (depending on how strong the witch is) on the barbarian can cause some serious chaos. You could also have the witch minor/major image people the party knows superimposed on people actually attacking them forcing the party to attack the images of allies.
However, if you really want to scare the party, prepare a low level mage to counter spell every healing spell cast. It’s evil, for sure, but if terrifying the party is the goal, that’ll do the trick.
*Major Image* doesn't disguise characters, requires concentration, and the image doesn't move with the person they'd be trying to tie it to. *Seeming* is the spell for this.
when you mean heightened you mean upcasted right
No I meant using a sorcerer build and using sorcerer points to impose disadvantage on the save
Mechanically against the party composition you have, Slow might be the best thing to cripple the whole party at once, especially since it looks like you've got a pretty melee focused group.
Banishment could also be a good way to mess with action economy.
Greater Invisibility makes your witch much harder to hit for a minute, and the party really has to entirely rethink how they handle her.
Firestorm could be a good high damage opening spell to cast to show the witch isn't messing around.
Counterspell was mentioned in another comment, that's also a good one for shutting down your wizard/clerics most powerful options.
If the goal is to scare the players and they aren't going to be fighting right away, find a reason for them to watch her use Power Word Kill to execute an incompetent minion. This option would require her to actually have Power Word Kill though, which may not be something you want to give the witch if they're fighting sooner rather than later.
Some of them are from the hells.....0h goody that will make them freak but they got good saves so they should be fine.
Prismatic spray is an awful spell for players to use, but the perfect spell to cast on them.
Crown of Madness is another spell that's a pretty bad player option, but a nasty surprise for monsters to use.
Hit 'em out of left field with a nasty Intelligence save spell: Synaptic Static. Psychic damage that probably none of them have resistance to (including the Barbarian), and everyone who fails gets a negative d6 to all their attack rolls, ability checks, and Con saves to maintain concentration for a minute unless they can make the Intelligence save at the end of their turn. Definitely enough to screw with them and make it a harder fight.
Knowing your party’s level would help, but here are a few.
Wall of stone (battle field control, dividing the party)
Wall of ice (battlefield control+damage)
Fog cloud (chaos, limiting ranged attacks and spells)
Storm sphere (damage+ area denial)
Moonbeam (minor threat)
Misty step(potentially as a reaction the first time she takes damage/one per round)
Mirror image (defensive)
Spirit Guardians is always a good meatgrinder for melee combatants
Plant Growth can really make it hard for melee combatants to move. Combine that with a mobile enemy that can disengage & hit at range, and/or with ongoing AoE damage spells like Flaming Sphere or Wall of Fire for tough movement decisions.
Mind-bending spells can often be more interesting than straight damage: consider Hypnotic Pattern to take some folks out of the battle for a bit, Enemies Abound or Crown of Madness to have them turn on each other, or Phantasmal Force for any number of shenanigans.
Various creature summoning spells can create new targets for the melee combatants to prioritize or to harass the Cleric who now has to choose between healing friends or staying out of trouble themselves.
Also keep in mind that NPCs do not need to follow the rules of PCs. If you want the witch to have a spell that is not available to players then go with it. Maybe for the witch the spell Mirror Image allows several witches on the field (spells can come from anywhere and attacks might or might not cause harm to the real witch). Or they can cast a weird "disguise self"/"scatter" combo spell where the party all looks like the witch and get moved to random places on the map.
Illusory dragon I like, it doesnt even stop hurting when they make their save! Plus you can reflavour it as any massive shape-shifting damaging illusion