I'll echo all of this, except maybe point #8. The mage is healing less hp, but his hp also goes further since he's taking less damage.

I tend to look at damage reduction as extra hp. If a guy with 1000 hp uses barrier then he effectively has 2000 hp. Suppose a MANGLE does 100 hp damage. Against the barrier'd guy it might do 50hp. When the barrier'd guy does "heal heal" at a healer, he's getting back 50hp. So regardless of whether he's barrier'd, he's still "heal heal"ing for a "MANGLE's worth" of hp.

This is different for built-in abilities like sanctuary for paladins/shamans/healers, since it doesn't nerf the amount of hp they get from external healing sources.

I would add to the above advice:

1. If you can't kill the guy, at least don't let him kill you. In a given engagement, if it looks like you have zero chance to kill him, just leave. Don't stick around to see how hurt you can get him.

2. As a corollary to #1, know what preps you need to have on hand in order to counteract the mage's usual means of preventing you from "just leaving". Quicksand+dispel/earthbind, bash, sleep/maledict, etc.

3. Try to catch them with their pants down. Outside of group fights (usually taking place inside or near a cabal), that's usually the only way you're going to take down this sort of character, short of...

4. Assassinate. Thing is, the kind of players who create "powerhouses" are usually pretty savvy when it comes to doing what it takes to avoid being assassinated. But...it's still out there as an option.