Neverwinter

Halfway up the mountain is the Triage Camp that has 9 trolls close by. You can quickly circle the area killing all 9 and then change to a new instance and repeat. Due to the camp fire being right next to this group of trolls you don't spawn far away.

You are limited to hopping instances every 2 min but this doesn't slow you down too much. I was able to average 9 kills in a little over 2 minutes.

216 kills an hour based on 9 kills per 2.5 minutes

Location of the camp:

The recently opened "Drowned Shore" is full of trolls. You will very likely get this achievement simply playing through the recurring quests there.

Icespire Peak
I have found two trolls in ice spire peak. When you get the mission to speak to Garora Oakhew. There are two trolls on the way up. I thought they might be giants but they are trolls. Tall and blue. 2 down 498 to go. Though when you get to the top of the ridge on the plateau there are quite a few trolls. I guess it is just a case of running round in a big circle killing them. Then the achievement should pop.
On the top of the plateau after speaking to Garora you will find 6 trolls in groups of 2. Also there is a single troll defended by 2 goblins on both sides making a grand total of 8 trolls. Kill em all is what I say. I am now up at 53 percent.
They are level 47 so hopefully I can level up and find more of them.

Manic Ado has provided this link


So thanks to him. Hopefully it makes it easier.

Go to Frozen Heart. The place is riddled with Trolls. You can easily get 9-11 percent every time you go through it.

You can also kill trolls by marching up and down Icepire Peak.

Drowned Shore also has a healthy amount of trolls.

Super easy with the Summer Festival event on right now (July 2017). One of the events is called "Hungry Trolls"... when this starts, dozens of trolls come into the area. Just keep killing until they are gone. I got 131 kills with my group in about 20 minutes.