Saitama has far more intelligence and has quite a bit more mobility.
(Speed =/= Mobility; speed can be equal, but that doesn’t mean a rhino can perform gymnastics.)
He can certainly outskill the beast and play a defensive battle until he can find a sharp rock or stick to beat Armsy down with, as the beast’s large, encumbering body will allow Saitama to perform better.
Saitama was able to beat monsters much stronger than him on his first go; this isn’t any different. Vote Saitama.