The Ascent

You will most likely get this achievement without even trying, but I will give some information to those who need it. To unlock this, you need to damage an enemy while they are trying to throw a grenade at you. When an enemy is about to throw a grenade, you will notice some orange lines around them (see the image below). At this moment, you need to hit them so that they will drop the grenade, after which it will explode on the ground and unlock the achievement.
I've also made a gif version of the image above so you know what it looks like when you make the enemy drop the grenade.

I think only certain enemies among the Joggers and the Turbo Thugs throw grenades (if there are more let us know in the comments).

• The Joggers can be found in the southern and southwestern parts of the Warrens, like in Gratitude, Nitroad, Blossom Hills or Gas Station. You will most likely encounter some of them first during the 3rd main mission Trading Places.
• The Turbo Thugs can be found in the Glut in tier: highStreet.

Below I have included a video of an encounter where I make an enemy drop his grenade multiple times.
I hope this helps. If I can improve upon this guide, please leave a comment.