For the valentines event where do I go to the defeat the Valentine’s slime and Choco Golem?

Those should be rare enemies in AtF/AfP.

Just do the rare spawn farming in AtF, master for the golem and standard for the slime

Just find one after doing AtF in Standard mode, that's the only place i saw one of them

For the Golem I did AtF: Master and the Slime AtF: Beginner. Both solo and I'd reset at the bridge if no rare spawned. The Golem took me the longest to get while the Slime only took a few tries.

I don't think difficulty matters, I encountered both of them in AtF/AtP (Can't remember which one) Master

I encountered a choco golem in Expert Flamehowl earlier. There's your other option.