This quest can be very easy or very hard ... it's all a matter of luck.
What you do is go to treasure island and right click ONLY ONCE on a treasure chest and one of three things will happen ... it will be empty, it will spawn a mob or it will say something about a glint of light or something like that. It is important that you pay attention when doing this as if you get this glint message you should immediatly stop what you are doing and f7 (to target the item) and then G to grab as if you right click on the chest again you lose the item and a mob spawns. Before you f7 - (g)rab the item it is important you get all members of your group very close to the chest so they are sure to get credit. If they are not right up on it, they don't get credit. I'm not sure how it was in the past but as of 05/03/2004 you must be right up on the chest to get credit.
It is best to go about this adventure with a sorcerer for crowd control. With CC you can do this with only a few people.
The mobs that spawn are not to hard for a small group of level 50s but if your group is unlucky enough to spawn a sentinal it is best to avoid it if possible (run from it to get out of range) unless you have a few tanks with you. Magic just isn't going to kill this thing so don't even bother trying.
The only other super hard mob would be the marble enforcers. They will be WAY easier than the sentinal but that isn't saying much. If you happen to run from these guys be prepared to run all the way back to the haven and expect atleast one of them to follow you all the way there. So do be careful with these guys as well.
We did this quest twice yesterday with a cabby, necro, a sorc and a buff bot. To get both shields it took a total of about 3 hours.