If you struggle against them, here are some tips:
1) Replenishment miracle (requires only 15 Faith, costs 30 FP) + offhand Gentle Prayer from any Chime (requires 10 Faith, costs 13 FP) = 11 HP/sec for a minute resulting in 660 HP for 43 FP in total.
Throw in a Blue Tearstone Ring and Sun Priestess Ring for good measure
2) Hidden Body (requires 15 Intelligence, costs 25 FP) + ranged attacks (my favorite are Greatbows) = cheese
3) Shield's wont work since there's no shield that protects for 100% of fire damage, and their weapons deal fire damage, which at 1 HP means you will die, so ditch the shield altogether and equip something more useful in the offhand, or just dual wield.
4) Hidden Body + (Spook | Slumbering Draconcrest Ring)
5) Roll to their side and not into/away from them.
6) Rapiers are very good against them, and also in cramped spaces such as Iriythyll Dungeon.
7) Black firebombs knock them down
8) You can walk behind and backstab most of them if you are patient enough.
9) You don't have to engage them, just walk behind them to places you want to visit.
10) Try not to engage several at the same time, if you do make sure you hit all of them with the attack (e.g. position them in a line towards you with thrusting weapons and in a sector in front of you with sweeping weapons). They are pretty slow so it shouldn't be much of a problem.
11) In the lower level of the Irithyll Dungeon where there are a bunch of Jailers, you can release the Lycantrope locked in the cell opposite of 2 Giant Rats and he will be hostile towards the Jailers.
Overall not a bad enemy, you can think of it as a temporary one-hit-kill mode. Playing DS3 for the first time with 10 Vigor and 10 Vitality I didn't even feel these guys as special since half the enemies oneshotted me anyway, even on NG+. My second pyro character felt the same on both normal and NG+. I finally saw where the hate was coming from on my third, shields-only character.
My problems are that the range of the drain and what triggers it is not clear. Also it would be nice if you had all the health returned if you managed to not get hit during the debuff, but I understand he current decision.