How To Slay Waterfiends
When I first had a waterfiend task I blocked them because I thought that they would be too hard, kinda like I did with skeletal wyverns, but after a little research on other sites I found out that they weren’t as bad as they looked. 
The waterfiends are level 115 and give 128 slayer experience per kill, they attack with magic and range. I recommend having 80+ in melee stats aswell as hitpoints, and atleast 40+ prayer and 70 range and defence for the karils armour. If you want to do these for training instead of slayer change the slayer helmet to neitznoit. I don’t recommend going to the waterfiends in the chaos tunnels, as it is multi combat, and waterfiends can be a handful with just 1v1.
(NOTE: to do waterfiends using this method you have to do some of the barbarian training from the barbarian on the lake outside the cave). I don’t recommend doing these if you lag alot, as they can kill pretty quickly if your not careful.
The easiest way to get to the waterfiends is to use a games necklace to barbarian outpost then put it in the bank in the deposit box inside the barbarian assault building. Then head south til you get to a pier with a whirlpool at the end of it. Enter the whirlpool, before you go down the stairs on the next level pot up and put your protect from magic prayer on to avoid the mage attacks from the brutal green dragons, and make sure you have a anti dragon breath shield or dragonfire shield on or you won’t make it to the waterfiends (you can drop the anti dragonbreath shield if you are only going to use 2handed weapons as there won’t be anymore brutal green dragons to worry about while in this area).
Above is the equipment I used to kill them, except for the sgs for specials, you can change the slayer ring to another ring if you like, I use this ring now if I have a long slayer task like this one was.
Once your ready to go down, head south past the brutal drags then west when you can til you get to the waterfiends, put on range prayer and your weapon, unless your using a 1 handed weapon of course. Waterfiends are weak against crush attacks so I recommend using a saradomin sword as it is fast and very accurate for the crush style attack. If you can’t afford to buy a saradomin sword, I recommend using a zamorakian spear as it is the second best crush weapon in my opinion besides the godswords, but they’re too slow for the waterfiends except they are good for specials (especially the saradomin godsword as it heals hitpoints and prayer with 1 special).
I usually take 2-3 bunyips (depending on amount assigned) 10 p pots 1 super restore/summoning potion, 1 super set, holy wrench, teletab or another quick teleport out, 100-200 swallow whole scrolls for the bunyips as the watefiends drop alot of raw sharks and lobsters and the rest sharks or tuna potatoes. I use berserker ring now instead of the ring of life, although you could use an explorer ring aswell as it gives a teleport and +1 prayer bonus, I recommend using ring of life the first time you go down there as it might be tricky at first.
Keep the range prayer on all the time until they become unaggressive, which is usually about 8-10 minutes of killing. Then turn on and off after you kill them. There is a log near the waterfiend spot that takes you out of the cave if you happen to forget a teleport. Some people take guthans with them to help, I tested it out and found that you get hit more with guthans on no matter what prayer you have on, if you do use guthans its best to use mage prayer as it doesnt have that great of mage defence but has a good range defence.
The picture above is from a 181 waterfiend task I completed a couple days ago, the godsword is a saradomin godsword which I borrowed from a friend, it works great there, rarely need to eat or use prayer potions. On this occasion I forgot to take the swallow whole scrolls, but it didn’t seem to make much difference, and I only took 5 prayer potions aswell. I accidently took 5 bunyips thats why there is 2 leftover in the inv still. The herb seeds were lantadyme and ranarr.
Some people range these when they get the waterfiends unaggressive, but in my opinion it just doesnt keep up with the rate of crimsons that melee gives. I haven’t tried it though, the waterfiends give heaps of mithril arrows so I guess it’s endless arrows there. Waterfiends also give heaps of crimson charms about 80% of kills have crimsons, and 90% are atleast a charm. They also drop alot of water orbs, water runes, mithril ore, seaweed, snape grass, blood and death runes which makes them very profitable and suitable for summoning and ancient supplies aswell as you can see in the above picture.
I hope this guide comes in handy for people wondering about waterfiends. I will add a map of how to get to the spot in the cave aswell as an inventory picture of what I take there.
If you have any questions don’t hesitate to ask I’m sure I can help you out.
Goodluck with your waterfiend slaying.
