When did Roshi fight Son Gohan?

[SSJ2]

Roshi claims that Goku took the Bankoku Bikkuri Sho better than Gohan was able to, so when exactly did they fight? When were both Roshi and Gohan that much weaker than even the 21st Budokai top contestants?

Also wouldn't a much stronger Roshi not try this technique on Piccolo Daimao?

 

[Hector]

Roshi had been holding back against Gohan, to avoid killing him, or hadn't realized how much power he had lost over the years.

Roshi knew Piccolo was beyond anything of this world (until retcons).

 

[Papasmurf]

Clearly before Roshi achieved MUI

 

[GSM123]

The timeline is a bit hazy, even Herms doesn’t have the exact details down. But we know for sure Gohan was around 90-100 years old, so he trained and fought with Roshi at least 200 years after the Piccolo Daimao Wars, presumably in a Tenkaichi Budokai.

Even the Kamehameha wasn’t invented yet when Roshi fought Piccolo (Piccolo doesn’t recognize it when he first Goku and thinks it’s the Mafuba), so the much more complex Bankoku Bikkuri Sho didn’t even exist yet.