i cant see namor doing anything that would hurt juggernaut. he took thors godforce and kept on walking. spiderman sent a truck into him head on causing a massive explosion that didnt phase him.
when he fought the hulk the first time he took alot of hits from a very angry hulk and his only reaction was to wonder if hulks mind could comprehend that he was truely unstoppable. the fight only ended when the x men showed up by fluke and jean attacked his mind.
also isnt namors strength level around 85 ton when hes in the water?
i just cant see many characters out there being able to take classic juggernaut out using anything other than magic or telepathy. i think it can be done, but i highly doubt namors ability to do it. hes fast enough to not get touched, but even if he evaded the juggernaut for weeks the juggernaut wouldnt be any more fatigued then when he started, and would eventually get a hold of him or connect a couple of times, which should stun namor long enough for the juggernaut to finish the job. looking at how easily hes taken out colossus i dont think hed have any real trouble.