First off, I think this topic belongs under Gaming Discussion, not Role-Playing. Role-Playing is a play-by-post area. Just for future reference.
Second off, I can think of a handful of spells that come in handy.
Bard:
0th:True Strike. You never know when you need to actually hit something and this helps with that.
1st: Tasha's Hideous Laughter. Assuming they're still intelligent enough to be sentient, this can shut an enemy down.
5th: Polymorph. Nothing quite makes the big dumb monster less threatening like taking away the big part.
6th: Otto's Irresistable Dance. See Tasha's Hideous Laughter
Misc.: Any of the "charm" effects (dominate, enthrall, crown of madness, etc.). Make your enemies work for you.
Wizard: as above, but...
Look for 4 types of spells
1) damage spells. This will let you hurt them. Nuff said. Fireball is generally the go to staple, but vary your element type in case of immunities.
2) evasion spells. These are spells like fly or invisibility that make the big dumb monster have trouble hitting you.
3) enhancement spells. Spells like haste will make your allies more effective at hitting their target, allowing you to get the drop on them and effectively reduce the danger to the entire group.
4) trick spells. Dumb monsters are less likely to make these saves so these spells are more likely to suceed. Avoid the "save or die" illusions like Phantasmal Killer though. These are spells like "silent image" or any of the bard spells I mentioned.
Otherwise, avoid debuff spells like feeblemind. If the spell affects their mind than it doesn't help because they already weren't using those abilities. If it affects their body, then they'll probably make the save.
So yeah... If you've played Pathfinder or 3.5; similar idea for what spells to pick in my experience.
