You have to be able to put out some serious dps in order to get them to 50% before they attempt to cast their heal.
The heal is not affected by WV at all, and if you use a stun to stop it, they will immediately start recasting as soon as they get out of it... The best way to deal with them is Silencing Shot since that seems to reset their cast timer CD, but if your dps is lagging, be ready to toss out a stun/scatter to stall a few seconds for SS to be off CD.. (Hate that they raised the CD to 24sec >_< 20sec would be perfect for farming these guys.)
Overall, they are not very hard to kill once you figure out the tricks they use ^^ GL
edit: Thought I should mention: Only took about 30min to farm up 3x of the tomes, so should be nice and easy to get your hands on. It vendors for 37g50s tho, so even after you get one you wont be upset about getting extras XD
The new raptor battle pets sell for 63g50s and are pretty common drops, so I could see this being a good place to farm gold during offhours too XD
Last time I tried it, I could get them past the 50% with one silencing shot, provided I hit it in time..
That was with a 510ish gear level.