You've encountered the hackers that farm the arena in DM, i myself have gotten rid of 4 by reporting them just in that area, super fast speed, teleporting in and out of the arena, good stuff....apparantly Blizzard really looks into these kinds of reports and takes action, i sugesst reporting them whenever you see them.....i myself was killed by one with the hacks , managed to hill the hacker, but he was able to retrieve his corpse in like 9 seconds. a level 60 hunter, who in no WAY can solo the mobs that spawn in there,  1 min run or so from the GY....he blinked out of the arena , healed up, waitied till skarr was almost dead, killed my pet, skarr and himself killed me.....im now on a mission to find these idiots.....sorry it happened to you, it really sucks.