I don't pretend to know everything about the class, I still feel like I learn something new about Defilers all the time; but these are some general tips that have helped me throughout my experience:
Always be aware of your surroundings and the location of both Freeps and Creeps during battles. Know where the Mini's/RK's stand, your Warleaders are, the Heavies and who they're bashing, and so on.
Don't stand in one place for long, as you'll shortly have Guards, Champs, etc. harassing your heals. Stay on your toes! Slow or fear them if they're consistent.
It can become difficult in big fights, but throwing out Flies + Curse of the Melancholic Heart on healers and Blight on top of condensed groups should be a part of your game plan also.
Don't be afraid to push into the back of Freep raids. Throw some HoTs on yourself, get into the back and drop a fear bomb on ranged and/or healers, whoever is causing the most trouble in your situation. You will become a main target, but your other healers (assuming you have at least a WL) should keep you up in order to make it back safely. This usually gives your DPS classes a window to strike on some squishy targets!
I generally cycle through my raid vitals for heals, though I like to keep certain Freep target callers selected so I can heal through that. It pays to know your enemy! Get familiar with Freep leaders and their target callers. It helps to know who you're grouped with too, and who is likely to be focus fired on (will it be your high-ranked leader calling the shots, or a Warg that Freeps have a vendetta against? lol). WL's and most Weavers probably demand the least amount of attention apart from when they're being focused on. Wargs are usually the class that are easily blown up, so keep an eye on them and your Reavers who will lead the charge. Try not to waste your big heal on a bubbled target unless absolutely necessary; just throw some HoTs on them and they should be fine. Put up Efflo' up at every opportunity.
As for out of group, as I said above, heal through Freep targets. But don't just focus on vitals, be aware of everyone on the battlefield and who is charging who, the Creeps that have over-extended, who the Hunters are firing at... numbers and green bars can only tell you so much, looking with your own eyes gives you greater anticipation!
Unfortunately I have no resources as far as info on Defilers go, everything I've learned has been through trial and error. Experiment with your Corruptions, there is no right or wrong - some prefer to be traited to survivability (max health + mitigation) and other like a damage/heal build. I still tend to swap builds every now and then, though I'm quite happy with Mitigation, Mastery, and Health. Playing a Defiler is no walk in the park, and can become increasingly frustrating. But if you stick to it, the rewards are worth it! I hope this helps a little mate; It looks like you're on my server anyway, so if you can catch me online then don't hesitate to ask any questions!