Hey I just got a dagganoth slayer task, but I have no clue where or how to kill them. I looked for a guide, but I didn't find a good one. Thanks

Most popular way is to cannon them under the lighthouse. No guide needed for that. There's a guide for slaying them in the chaos tunnels located here.

Cannoning at lighthouse-15-20 min quick task, you'll use like ~1k cballs, annoy some players, but who cares!

Lighthouse-Fight them with melee, easy task since it's multicombat + combat familiar/bunyip and semi-afkable but a lot of people and cannoners will come too.

Waterbirth-single combat, hit somewhat hard, guess you'd earn money but task takes a while

Chaos tunnels-Gotta pray, multicombat so combat familiar is recommended, I did it once, took 50 mins or so and I lost money, although you're supposed to make money.

 

Then again, all I do is cannon so I'm not so sure on the other ones.

EDIT: and DKs but never done them before and don't think you'd want to if you want a fast task.