I’ve personally found great effectiveness using cloud of daggers maybe more than any other spell. Funnelling enemies through a passageway with multiple cloud of daggers cast on the spot is probably my current meta. Then I utilize thunder wave/ black hole to keep the enemies on the other side of the daggers once they make it through.
Another thing if found amazing success with is getting a party to strength 20 as fast as I can, which then I find 90% of enemies can be thrown off cliffs for instant kills. Throwing enemies off cliffs also only requires stealth checks where the body hits the ground and dies.
Lastly there is a fun little cheese strategy I’ve picked up that I don’t see anyone talking about. If you cancel an improvised weapon attack with an NPC before you deal damage, you can move NPC’s without angering anyone.
My favourite tactic is based around Hunger of Hadar, a Warlock spell that creates a zone that a) deals damage to everything in it at the start and end of its turn; b) is difficult terrain (aka slows everything down); c) blinds everything in it; d) best of all, has no save. Plonk it down in the middle of the enemy team, use another control spell to slow everyone inside even more, use shove/repelling blast/whatever to push anything that’s made its way to the edge back to the middle. Wait for everything to die, rinse & repeat.
But the greatest cheese in the game must be casting darkness (or shooting arrow of darkness) on your own party. As long as you’re inside the cloud you are basically untargettable by spells and ranged attacks - and the enemies don’t seem to be smart ebough to be throwing Fireballs on just any weird clouds they see. On your turn you just need to step out, cast/shoot, step back in. The only way they can get you is if they come inside the cloud with you (aka in range of whoever is on blender duty, Lae’zel or Karlach like this posting a lot), and the AI gets a bit confused when it can’t see you so it’s not a sure bet that they’ll even try. It’s a thing of beauty.