Okay, now you have mastered the Stump's Weakness skill, right? Now let's use it to hunt some Stumps.
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 Stump's weak points? Try eliminating around 80 of them!

Okay. Eliminate 80 Stumps.

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 Stumps.
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 80 Stumps yet.