Borderlands 3

First thing you want to do is go to the closest ammo dump vending machine and purchase a grande mod that specifically has Sticky listed on the traits list and equip the mod from your inventory. You should then go out into the wilderness to find a group of weak enemy's (for example I found a group of skags). throw your grande into the general area of the enemy's (this does not not need to stick onto one of the enemy's) wait for the grenade to go off and your achievement should pop. If it doesn't then try again until it does.

Side notes:

You can damage enemy's and still get the achievement as long as the grenade gets the kill.

As noted in the description you can get more than two kills from the grenade you just need to make sure it kills at least two.

If you would like to see a video guide for this please see the one I created below.

In this video guide I will show you how to kill two or more enemies with a single sticky grenade. Now first thing you need to find a grenade type that is sticky. You can either get one from buying in the vending machine, by killing enemies, or loot chests. Once you acquire one you will need to find a few weak enemies to round up and kill. Like some skags or the ratches will do. I did mine one a side quest for a guy to clear our the cellar. Once the bug went into larva mode I rounded up the second bug and threw the grenade in the middle. If the explosion got the finishing blow on the two, the achievement will pop.

If you are like me, and missed this achievement (or picked Zane, and didn't use grenades), there is a simple way to get it.

- Acquire a sticky grenade.

These can be bought from the Ammo dump. If you've completed the game, set Mayhem Level to 10, reload your game, and visit an Ammo dump, Sanctuary will do. With the Mayhem 10 modifier, the grenades will have high stats.

- Load your game on normal mode, not TVHM.

The enemies wont scale to your level on normal mode, so you can easily out level them.

- Fast travel to The Droughts on Pandora, and head north past the vending machines. Over the road you'll encounter a Skag nest, enemies about level 8.

- When 2 Skags are close enough together, throw the grenade (Doesn't have to stick, but the grenade must deal the final blow to 2 or more Skags simultaneously).

This can be attained early in the game whilst on the planet Pandora.

Find an Ammo Dump vending machine and look through it's contents until you find a grenade mod with the word 'Sticky' in it's name and purchase it. If you can't find one you can reload your save and return to the same vending machine and the inventory will be different.

Equip the new grenade mod and head out to find a small group of low level Skag pups. Weaken a few of them with your pistol and when they are low on health, throw a grenade in their vicinity and if it takes out at least 2 of them it will unlock the achievement.