You'll hate me for this, but he doesn't beat anyone.
It was pretty much confirmed by Future Trunks vs Goku that the SSJG = SSJ statement by Beerus was either retconned or forgotten. Goku's Saiyan Beyond God level is now a completely different form from his base, so Frost > Base Goku =/= Frost > SBG Goku.
In my opinion, Piccolo has reached a complete plateau. The fact that even after seven years of training from being able to take on a Cell Junior and hold his own for a time, he was much weaker than Kaioshin who was weaker than Perfect Cell means he's not gaining much if any power from training anymore. Considering what I said above, there's really no evidence at all that Piccolo has gotten much stronger than he was in RoF, and he's certainly not at the ridiculous, fusion-beating heights some people think he is. Yes, this means everyone in the U6 tournament except Hit was pathetic. We knew that already. I doubt he's gotten the power to beat Tagoma.