Trust me, the nice people who would actually take time to explain the dungeon are really out there. I'm one of those people. I even taught a tank how (s)he could mark mobs, which skills (s)he could use to generate more enmity and how certain boss battles work in the first 3 dungeons (and i'm not even a tank myself).

I would ask the rest of the party to let the tank mark the mobs and have him/her get the first strike, so that (s)he could get a feel for it.

I would either join these groups as white mage or black mage, so that I had control over the battle with cures or sleep.

I believe it comes with the territory when you know the dungeon by heart and wish to assist in helping the community get better. But, every community has some "bad apples", this is something we cannot avoid. However, we can assist those who genuinely seek help and wish to improve themselves.