Well the reason for this is 99% certain that he use Bloodthirst and/or Beserk that affect others and get the debuff that cant hurt the boss and therefor times out or eventually dies. If you only use the most common 5-6 attacks this is not a problem for a marauder. I did it with less than 15k health, No problems
Probably. Marauders have a couple of skills that they commonly use that will give them the damage debuff and then they can't possibly finish (either party healing from DOTs or damage boosting the party). Anniliation has this issue often if the person isn't thinking/is new.
A marauder with 15k HP should be able to do it w/o any serious problems, especially with help.