Staff of Magnus: Boss fight- Stand next to Magnus as soon as you enter the chamber. Make sure you have a follower with you, kill the top two mages with a ranged attack. Do not kill the bottom and third mage, let your follower kill him because Magnus is still focused on the third and final mage leaving you to attack him without him fighting back. This made him very easy to kill instead of his usual flying around the room.

Coral Claw

The Coral Claw is obtained from http://elderscrolls.wikia.com/wiki/Birna, the shop keeper in http://elderscrolls.wikia.com/wiki/Winterhold_(City). She claims she purchased it as it would lead her to great treasure, but that never panned out for her. She offers it to the Dragonborn for 50 septums in exchange for telling her if it ever "works" in the future. She also tells the Dragonborn a possible location, the http://elderscrolls.wikia.com/wiki/Yngol_Barrow. Located Northeast of Windhelm. It may also be found at Yngol Barrow if you have not talked to her yet.

-If you chose to talk to her, make sure you do it before you start the College of Winterhold quest because her and her brother will die when a random dragon kills them flying over town or over the college. Her and her brother will attempt to fight the dragon and they will be instantly killed.

General quest tip : Don't kill anyone, enter any location or pick up any item until you have a quest to do it.

Note : Even following this tip, there's no guarantee that the quest still won't get borked.

No point in returning to Birna to tell her what happened, btw. No convo options for that.