I just camped the tiger and here is what I figured out.
I spent about 2 hours here and killed the tiger about 25 times and he didn't drop the tiger skin once. It was getting late, so I camped my guy for the night. First thing the next morning I got on and killed the tiger and he dropped the tiger skin on the first try. Then I just gated back to my home bind point with my newly acquired tiger skin.
I found that standing in the back corner behind the tree works very well. The guard stood up on the deck and just watched me kill the tiger. He never once came after me.
The tiger does not run away. Every time I nuked him (careless lightning), he would try to run, but would stop at the ledge and not go any further. Another nuke and he was dead. So there is no need to waste time and mana to snare or root him. Just kill him. I guess it is possible that someone tried killing him with a spell that causes him to jump backwards (such as whirling wind or dizzying wind) and maybe that is how he got off the ledge. But as long as you use a DD spell that does not cause him to jump back (or melee him), he will not leave the ledge.
I was also able to stand in the back corner and melee him until he was dead. Both ways worked just fine (nuke and melee). As long as you stay back in the corner behind the tree, the guard will leave you alone.
And the tigers all conned green to me as a level 20 druid. I never saw JoJo.
Finally, according to what the cat at Kerre Isle says, you have to get a tiger skin from a tiger not of that island. In other words, it sounds like you cannot kill the tiger on Kerre Isle and complete the Bone Talisman Quest.