Guild Wars 2 Bat Slayer Achievement Guide
Bat Slayer is definitely one of the Slayer achievements you will not get from normally playing the game. Very few quests, events or activities actually involve you killing bats so getting it over the normal course of events is very very low. Without killing bats for any other reason than normal ones I have a total of 498 / 1000 kills which is actually probably more than what most players will have (considering I am a super nerd).
There are two main places for players to farm for these bats, the first is in southern Plains of Ashford. I actually don't really like this farming location because the bats drop Vial of Weak Blood which doesn't sell for that much. I prefer hunting the bats in Gendarran Fields because they drop Vial of Thin Blood which sells for 3x more than Vial of Weak Blood. Although you will be able to kill the bats in Plains of Ashford a little bit faster (literally maybe a second to two seconds faster per kill) you will make triple the money farming.
Honestly, listen to me when I tell you that you want to make more money farming versus spending less time doing the achievement. The amount of things that you accomplish per time spent equals out to be so much better if you do Gendarran. Below is a picture of the location in Plains of Ashford. As you can see the bats can be found just south of Lake Adorea. There are quite a few here but if you're a high level player you definitely will be waiting on respawns.
The second location is just north of Lion's Arch in Gendarran Fields. You're probably familiar with where it is if you've ever visited Milton before to buy some cooking ingrediants. The location is a hop-skip and a jump away from him just to the west. Just north of Broadhollow Waypoint you will find some bluffs, all along the edges here you'll find the Cliff Bats to kill. There are plenty here for you to kill, but if you run out for whatever reason while you're grinding here you can always kill the Grubs nearby for Insect Slayer or the Raptors for Raptor Slayer.
Also a bonus for being a higher level here is you will get level 80 loot for farming here, sometimes at least. If you’re familiar with how things work in Gw2, when you’re a higher level than the content you’re doing it will half of the time drop loot your level and the other half of the time drop loot at the level of the content.
In closing I strongly recommend you do Gendarran Fields. That part, by now, is obvious ;D
Though if you would like to see a video of my typical farming path here, feel free to check out the embedded video below! That's all there is too it, best of luck to you with getting Bat Slayer!