Jinbe's best bet would literally be to drown Sanji in the 2nd round.
One thing I think people have been taking for granted recently is Sanji's endurance. Yes, Jinbe has the means to bypass Sanji's newfound exoskeleton, but that does not mean Sanji is unable to withstand the pain or damage done to him. He's taken major damage that's broken his bones or wrecked him in some way, and managed to keep fighting/moving even Pre-Time Skip. He hasn't suddenly become Cracker, who can't take pain at all. On top of that, there is nothing implying that the pain he feels is lessened in any way by his exoskeleton, given that he could still feel the pain of being shot in the head by the bullets of fodder.
So, taking all of that into consideration, Sanji, after having all of his bones broken by Queen, was not only able to remain conscious through that pain, but got up as soon as he realized he could still move. And that's before his regen kicked in. At this point in time, Jinbe hasn't shown anything that makes me think he can deal damage to Sanji near the level of Brachiohiss, which isn't even the threshold of what Sanji could potentially withstand.