When I first did Withered training I was moaning about it. Doing it as resto was really poo and I didn't have my full 2k mana then either, so I had a terrible score. It was not particularly encouraging to listen to the tanks tell you how easy it is. To me it was a bit like those treasure runs in Throne of Thunder - I was lousy at those too.
Having done it 4 times now, I have improved each time, much to my delight, and now I am no longer dreading it or hating it like I used to. My first score was 62, my second was 100 or so.
I had my first encounter with Leystalker Dro on my 3rd attempt, which was the first time I was feeling rather good about myself as my score this time was 325. That was also the first time I had 20 withered and I had gone boomkin to try to kill off those exploding guys which take out a lot of my withered as my interrupts are a bit lame!
I listened to everyone talking about how they killed Dro and resigned myself to making sure I just got as many chests as possible because I probably wasn't going to kill him with my lame DPS. However, Crooked told me something which was super helpful!
When you talk to Thalyssra and give her 50 mana you will get the buff, Ward of the first Arcanist. So when Dro killed me, I got a bubble, and when Dro hit the bubble it did so much damage that Dro died! Crooked said that he thought Dro was at 50% when Crooked died and got the bubble and then Dro insta died. I tried it for myself and I had Dro at 46% when I died and I can confirm that Dro DID die instantly after that trying to hit my bubble!
There are tons of guides out there about how to get the most out of your withered training, so I won't be talking about that here, but I think even as a healer with that buff you could get through the training a lot easier than you used to. I am yet to try it as a healer, but maybe I'll give it a go when I have no further need of all the rep items and army buffs.