[Tyranitar140]

Im not sure which mobs, but many mobs are healed by instant damage or poison potions and are hurt by healing potions. Try using a healing potion or instant health potion.

[Sharp_Enderman]

I can also hit the freshly spawned, not bunched up spiders with the potions perfectly fine

Zombies are damaged by instant health. Spiders are immune to poison.

[monkeychunks]

Don't use insta damages just make a crusher.
P.S. If you think you're having grinder issues try having 5 spider spawners, and a zombie spawner within range...

[Sharp_Enderman]

I have a crusher. The potions are to finish all the spiders in one fell swoop. On that note, I've pretty much settled on just punching the spiders due to my frustration with this issue. If someone ever comes up with a solution then I'd still be very interested in hearing it.