If you are getting a boss to within 5% have you tried going in with consumables? +spell damage food and elixir can get you ~100sp damage which might put you over the top.
http://www.wowhead.com/?item=22839 might help you also
Also are you equiping 2 clicky +spelldamage trinkets. I think you can pop these back to back so you are doing more spell damage for a longer amount of time. ie 30 seconds of the spell damage boost vs 15 seconds of the spell damage boost.
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