Okay so there's a Marlix at Waste E, (head south) on Smp2 that should be dead by now, all the armor has been knocked off. Three of my friends were fighting it and they asked me I wanted to join. The coords for it are 3933.209/68/227.439, its around that area. I'm not sure it's a bug though because it might just have a lot of health.
Well at this point, only time will tell. Just keep fighting your little minecraft hearts out. Take down this super healthy Marlix with the flu or perhaps just sneeze on him.
Chances are your difficulty level is pretty high which makes it harder to kill the Marlix. Good luck killing it though!