From the experiences I've had in 10+ leves it seems you are always fighting two or more monsters and that people aren't holding hate very well so I thought I'd point out a few things.
Glad - > Provoke provoke type skill at lvl 10
PUG -> Taunt provoke like skill at lvl 10
Mar ->Disorient Provoke like skill @ lvl 18 (!)
Glad makes the best tank because they can wear heavy armor and use a one handed weapon and shield. Since you are fighting more than one monster you can either have one tank for each monster and have them keep the hate really well, or you can have one tank and have them taunt one monster and provoke the other. I doubt anyone trying to tank has disorient AND taunt and Provoke yet, but having provoke and taunt should at least be obtained for group tanking. Also, monsters seem to HATE healing and buffing... so you could throw in a CON lvl 4 to heal yourself some during battle.
Now, the healer should stand between the two tanks (if there are two) and next to the tank (if there is one) so his heals can hit others if they take any damage, which they shouldn't if the tank doesn't lose hate.
Almost forgot... for a tank, upgrading your armor is essential. Don't join a 5 star level 20 leve with Weathered equipment on and try to tank...
Thats my 2 cents, anyone else feel free to add or disagree.