The best place to grind this is during Chapter 22 - The Dreamers of the Day. As you progress through the level, you will come to a bridge between two towers that you need to cross. There are several places to take cover on the bridge and a load of enemies on the other side that will toss lots of grenades your way. On the side of the tower on your side of the bridge will be a ladder you can climb. In the room upstairs you will fins a T-Bolt Sniper rifle, so grab it and go to town on the enemies down below. As a bonus, any grenades the enemies throw at you will go harmlessly down through the hole in the floor.
You can reload the checkpoint as many times as you need, and you'll start at a point just before the bridge so you can try it again.
This is also a great place to get the following trophy, so see my other solution for that:
Note: This trophy will unlock immediately after getting the 30th kill.