The best to use the LPU for mythical upgrading. The LPU are usually maxed out. The EPU, are maxed from the Clan Shop. From the campaign, they are not maxed. I max out the EPU with RPU, CPU, and other items. I like to save, so I mess around in the upgrading mixing stuff. These EPU and CPU are the same lvl, yet different stats. I got them from completeing bosses.
I will show you the EPU difference. The EPU At lvl 20, the boost is 59274. The RPU is 50k. Right now the EPU is at 35,390 ....
To get it maxed, you have get it to 82,270. That's 46,880 boost I need.
You can use the the RPUs for the EPU or just random Items.
It's best to use the EPU for Mythical upgrading, also. You can also use the EPU for legendary upgrading Just use the LPU for mythical upgrading, nothing else.
I've just made a new account and I will be posting my progress here. I will tell you most everything that happens and hope for tips along the way.
I won't have set days because I will be off and on.