Comments posted to our Dark Souls 2 Wiki
Watch out, the page doesn't list that they're immune to toxic and poison.
I can confirm that they aren't immune to poison. It was absolutely painful, but I managed to poison one of them while dodging and weaving. It's not you but it took me maybe 15 arrows at a reasonable pace. They may as well be immune as it's not worth the hassle. Just get under/behind them and slice them with Dark.
Watch out??? xD Thanks for the PSA.
I got out of the way just in time.
I find the best way to fight them is to lock on to one of the legs and keep circling close in that direction. It will try to turn to face you, often not attacking at all, and the bite attack and head lunges will usually miss. The main danger is the stomp attacks and the jump. When it starts stomping, roll away and move to a safe distance (it's a good opportunity to heal if needed) then immediately move in close again. The jump is a bit tricky due to the strange timing - you basically have to roll slightly later than you'd expect, practically at the same moment it lands. I have no idea why that is.

Shields are fairly pointless against these guys so just two-hand whatever weapon you are using and keep taking shots at the legs.
Resistant to Lightning? Well if they are the fact that they are in water makes spells like Heavenly thunder and blinding bolt ruin their day. Cast it under their head and it will do almost half their health on NG with one cast.
Seeing that people have troubles with these fake drakes, i ll give a trick for easy kill: Upgrade crypt blacksword to +5, imbue it with darknight stone and buff it with dark weapon. Even in the covenant of champions they go down in a few hits Pd. Tested this insane dark damage with 40 faith and 40 int. Try it and smile if it works for you