Weapon Mastery can be one of those more annoying achievements, up there with Giant Slayer in terms of time and dedication needed to getting them (excluding PvP and WvW achievements). Not to mention the fact that shield and warhorn are some of the must frustrating weapons to get kills for. With this guide we take a simple route that only requires time once you get there.
Where you want to be to get started on truly grinding out these achievements is Forewarned is Forearmed (level 48 personal story), the Vigil part of the storyline. You'll want to be about level 45 if going solo, but a higher level friend can get you to this part of the story with ease (my guardian is currently level 26 and doing it). This particular part of the storyline will put you underneath Lion's Arch and the most important part is that you will face a never-ending stream of rats that doesn't involve you moving much at all.
I would take a Guardian, Engineer and Ranger to cover every weapon. Sure you might have to wait for a cooldown once or twice, but you can remedy the wait by netting a few kills with your main hand weapon or bringing in another class to cut down the wait for your weapon. The benefit to bringing a different class is that you can kill rats more swiftly, the downside is that you'll either need to level another character to about 45 and get them through the Vigil storyline again.
Alternatively you could just be going for the main off-hand weapons of: focus, shield, warhorn and torch. If that is the case I would suggest only bringing a Guardian and Necromancer to become rat-killers. Guardian's shield and torch are great AoEs (although Zealot's Flame doesn't count as a torch kill unless you throw the flame at a single rat), and the Necromancer's focus and warhorn are some of your fastest options for just those weapons.
What To Do
This is the easy, albeit the most tedious part. Plant yourself at a sewer bend right before the rat spawn point, face the stream of rats and get killing. If you go the Guardian, Engineer and Ranger route you'll find some weapons take more time like Dagger and Focus, but you can cut down that time if traited appropriately.
If you are taking the Guardian and Necromancer off-hand only way I suggest taking any weapons out of your main hand so you won't be accidentally swinging your weapon or anything. Just pure off-hand damage going into slaughtering rats by the thousands. Make sure to turn off auto-targeting in your options otherwise you will start attacking things underneath you in the water.
Finally if you are underneath level 45 the easiest way to handle the bats and ooze you will run into is to take off your armor and death zerg it. This will keep Forgal alive (as you will keep aggro) to deal with the bats and ooze while you keep spawning and dodge rolling around the mobs. Make sure to pop heals and any boons you can give Forgal while doing this.
What About Underwater Weapons?
If you are intent on grinding these out I would say to go to the Wayfarer Foothills and go down up and down the path in the screenshot above. This path will take you through a whole mess of barrcuda, alpine skelk, ice drakes and salmon that are on about a one minute respawn time and you will be seeing about 50 creatures, under level 10, on a full circuit. Doesn't get faster than that to wrack up the kills, in my book.