Alright, I've seen a lot of new info regarding waterfiends, so planning again. Will update this post if / when necessary.
Equipment (cost: ~7m)
Helm: Helm of neitiznot.
Body + legs: Karil's.
Boots: Dragon boots.
Cape: Fire cape.
Weapon: Zamorakian spear (crush).
Ring: Ring of wealth (try it out!)
4-5 bunyips, with a few hundred swallow whole scrolls.
Fires + nats, for alching.
House teleport, just in case.
Enhanced excalibur (elite).
Fairy ring code BJQ goes straight there.