Disciple is most likely the most top heavy healer right next to the the guy that makes elixirs. As the OL I get most frustrated with those two classes.
But Steve-o is correct. The healer needs to be the target of your wrath with a shower of attacks or pulling him away from the group. Something I am learning to do is take out someone (normally the scout) by pushing them away from the group. and then punch... allot. If he is removed group the group (and often it's natural for the scout to want/need to run off of their own) then he becomes a card generator for the OL.
Stun is beautiful. With your expansion will also come with Basic II. Overwhelm is a good card if you can pull it off. Either effect is nice. If both immobilize and stun is inflected then the healer can only take one of the two off. If you are able to put a status aliment on more then one hero, then the healer has to decide who to help. The spider group you get in this next box are fantastic at doing this! Also, harpies, found in trollfen, is the prefect group for this.
If you get trollfen, you will also have access to Infector OL calss. I really do believe Infector is the answer the OL has to hexer and hard to keep up with healer classes. It's also a great way to deal with well armored heroes. I had a hero walking around with gray black defiance dice and two shield (and the hand bow. It was funny). A large, wimpy open group rained down on him with no damage done. The infection tokens built up till he was ripe for Outbreak. That hero failed his might check and went down because of his unhealthy stack of infections. He lost his turn which was bad since he was the healer with a hurting party.
There are ways on dealing with your healer. Sounds like the heroes choose powerhouses of characters and classes. Hang in there. Look for weaknesses and holes in their plans. And if you are playing LoR next, get that sunstone as soon as you can. I never needed the exp it could give me. There are allot of LT that can use it. The extra green dice is really handy. Ariad had it for all of the final Rolling three green made is far better for me to use her more up front in battle.