Usually best for helping complete fusions during Champ Chase and Summon Rush. 

 you could pull them all during a champion chase/summon rush event and feed all the 1 ,2 and 3 stars into each other, very tedious process of course.

@TuckerG   I have used 3500 for a fusion event.  Its not that difficult once you get into the rhythm.  Be sure that  you check for the rares that you should save for SR in DT for any other fusion events.  

Thanks all much appreciated