Okay, now you have mastered the Slime's Weakness skill, right? Now let's use it to hunt some Slimes.
1) How do I use this skill?

There's no need to worry about that now. The skill will be applied regardless of the type of attack you are using. You see the Slime's weak points? Try eliminating around 60 of them!

Okay. Eliminate 60 Slimes.

If you don't want to do it, I have to say, it's your loss.

Okay, okay. Great work! You look like you can see and attack the weak points of the Slimes.
1) Do other monsters have their weak points as well?

That is an excellent, excellent question! Monsters' weak points have yet to be revealed to everyone, but I do believe they will become known someday. I was able to find the weak points of these monsters, but I didn't train hard enough to make myself a famous adventurer. But I have a feeling you have the potential to achieve much more than that.

Keep training. Don't stop until you become a famous adventurer!

I don't think you have slain 60 Slimes yet.