If limited to his regular power, I'd say Namek Galu (post-Zenkai) only needs a Kaioken x2 to stomp him.
If the Shin Kikoho is allowed, it ought to be stronger than the version that caused some burns to 2nd form Cell, but probably not by much. Tenshinhan seemed to be in admittance of having hit a wall and regular Earthly training isn't going to be enough to make any significant gains, if any at all. The Shin Kikoho ought to be capable of taking out #16, but anything higher is very unlikely. At best, maybe he could deal significant damage to initial 2nd form Cell.