As I said above hes not done it before, but technically Kain is dead in most ways you would assume to think someone is dead, hell hes not even got a heart and the only part of him that "may" be touchable is his soul which is what I would assume Elder God would use considering thats what it eats and uses to spin new life.
now Kains soul is untouchable by the EG, all vampires are. He needs agents like wraiths (raziel) to process the soul for them but even then Kain seems untouchable as the writer of the series claims the only reason Kain is alive because of his "nature as the scion of balance.
I think physical thrashing is Elder Gods best bet of winning and expanding until its mass is ridiculously large. If thats not enough then nobody wins this match, Sin is too slow and not powerful enough to cause enough damage to EG size even if he is allowed to hit him.