Load up Act 1 - Ashes, chapter Fish in a Barrel and kill some enemies around the first Troika. You will get a checkpoint right before you enter the area where the achievement is, so do not worry if you do not get it the first time.
As Im LoganXP pointed out, it can be done in co-op.
You will have 2 grenades coming to the fountain, there are 2 more inside, so there is no room for error.
Every time an e-hole is created, you can focus it with pressing the Y button. You need to throw the grenade RIGHT WHEN THE HOLE IS CREATED. If your grenades kill enemies, that are emerging on the surface, it does not count, even if the hole is sealed.
First three holes are super easy, if you throw the grenade right as they emerge, but the last one can be tricky, because you have to throw the grenade as the cars explode, do not wait with your grenade, it's explosion has quite a range, so even if you do miss a bit, it should count. I personally threw all the grenades from inside the fountain, so you do not need to panic, just focus on the directions.
If the chievo does not pop as the objective is crossed on the screen, immediately pause the game, wait a bit just to be sure and select Load Last Checkpoint, which will put you right before the fountain, so you can try again.
Good luck!
Here is a video, made by Skycaptin5, to help a little bit more.