How about mega potion of intellect?
It give a 15sec small burst but it take 5MIN CD
Normal I will combine it with the raging strike to maximize damage output.
But I found that using potion give me about 0.2sec delay before i can cast spell.
So now I start using below rotation on raging strike:

(in UI3 buff) thunder II/III > Fire III > potion > raging strike > Fire I > ETC.
After casting Fire III, I have some time to use the potion and cast raging strike as cast time of Fire II in UI3 is less than recast time