Start working on all those "worthless" level 1 characters on your roster. You burn through Mk5 power cells at an alarming rate. I ran out, even. (But that was when you didn't get any in the 5th tier, they changed it shortly after)
Eventually you break through that point and start stockpiling again. I think I'm at about 500. I figured that was a good enough number to be at, so I stopped doing the tank challenge. Don't sim it, don't auto it. They've got metrics on who does what in game, if they see TAC Challenge participation drop they'll work on it.
In the meantime--don't sweat it. It's not mandatory.
