From reading Himeuta:

The drop rate for senki in the dojo is normally abysmal on any floor other than the final 10. And you can't boost it using luck like when farming inhumans, because the luck bonus reward for completing a dojo floor is a daruma.

You have to use your extra copies to evolve them, like how you use inhumans for godifying 5* girls. DO NOT COMBINE THEM USING THE LEVEL UP OPTION UNLESS YOU NEED TO FULFILL A LUCK REQUIREMENT.