While Roshi was the martial artist responsible for creating the Kamehameha technique–one so powerful that it can literally blow up the moon from Earth’s surface–that original move isn’t the most powerful attack in his arsenal as Roshi proved his strongest weapon is actually the manipulation of Goku’s most terrifying Saiyan form.
When Goku first landed on Earth after being sent away from his homeworld of Planet Vegeta due to the planet’s impending obliteration, it was immediately obvious that he wasn’t like other children in that he had a monkey-like tail, which is a decidedly abnormal additional appendage. While it seemed like a simple cosmetic difference between humans and Saiyans, it was soon apparent that Goku’s tail was the key to one of his most terrifying forms: the Great Ape. Under the light of the full moon, Goku’s tail granted him the uncontrollable ability to transform into a monstrous, rampaging beast–and Roshi found a way to use that powerful form to his advantage.
In Dragon Ball chapter 119 by Akira Toriyama, Master Roshi is participating in the Tenkaichi Budokai and has come face-to face with Man Wolf, a werewolf-like fighter who has a serious grudge against Master Roshi. In the previous tournament, Roshi destroyed the moon in order to stop Goku while the Saiyan was in his Great Ape form. When he did so, he unwittingly cursed Man Wolf to a life where he can never turn back into a human as his lycanthropic curse is the opposite from that of a normal werewolf. Rather than being a human who turns into a Wolf Man while under the light of a full moon, Man Wolf is always a humanoid wolf who turns into a human during a full moon. Since Roshi destroyed the moon to neutralize Goku’s Great Ape, however, Man Wolf can never be human again, which made the villain murderously angry. However, Roshi came up with a perfect solution to Man Wolf’s problem–one he could seemingly use on Goku as well.
Using hypnosis, Roshi was able to trick Man Wolf’s mind into seeing Krillin’s bald head as the full moon, and due to the immense power behind Roshi’s technique, Man Wolf’s mind believed it and permanently transformed him into a regular human once more. Based on the success of Roshi’s manipulation of Man Wolf’s mind, it seems possible that the martial artist could accomplish the same thing with Goku. If Roshi could make Goku transform into a Great Ape whenever he wanted, then he would be absolutely unstoppable as that Saiyan form has the destructive potential to level an entire planet in a matter of hours.
The only problem with this hypothetical battle maneuver is that Roshi himself could very well be killed by the rampaging Great Ape Goku, and Man Wolf’s transformation proves it. When Roshi hypnotized Man Wolf, his transformation into a person was made permanent, meaning Man Wolf’s mind always thinks there is a full moon. If that was the case with Goku, then the Saiyan would never revert back to his more human-like form and would potentially lay waste to the entire world.
Regardless of the consequences, however, Roshi seemingly proved that he can make Goku transform into a Great Ape anytime he wants–effectively making Goku a living weapon much more powerful than the Kamehameha.