The Elder God could neither be seen nor touched without Kain gaining Raziels healing "purified sight" and the soul reaver, the Elder God is by its very nature untouchable without special means but assuming he fought the Elder on the same terms Kain did, he should be able to do the same as Kain.
The Elder is huge and has a lot of physical might but its static, relatively slow and does not use all its resources/physical ability at once.
I don't think so because if you recall Raziel used the wraith blade, the most well known soul devourer in the series to try and hurt the Elder God and it laughed at him. Not sure if the Elder God itself has a soul, hard to say., I mean he eats souls and spins them in the wheel but whether or not he himself is soulless or not is undisclosed.
But no, I think you need to use a special sort of weapon to hurt the Elder God, Kain was lucky he had both the sight and the capability to harm the elder through a series of plot specific events.
Maybe if you gave Alucard purified sight like Kain has he could harm the Elder, in which case he can probably win. If the Elder gets a hit on him though with all those tentacles its going to go badly for him, his soul will also likely get eaten.
Well Kains soul reaver has the magic of the actual blood reaver blade as well. So their sort of combined, Raziels wraith soul and the ancient vampires magic of the blood reaver. It would seem that combination is enough to hit the Elder God.
It could also be to do with Kains purification, maybe normally nobody can touch the Elder but in Kains case, he is special. Afterall, he is the scion of balance and the balance guardian in one.
A number of factors lead up to him being able both see the Elder God and hit him, Raziel could see him but not hit him with his soul devouring weapon so Kain had that extra advantage of the blood reaver.
Remember the completed Soul reaver as the primary weapon of the series has always done crazy things anyway, such as form paradoxes (or quench them) and is further enhanced with all the emblems meaning that when he brings it down upon the Elder Gods form he is hitting him with not only Raziels wraith powers or the ancient vamps powers but also that of the elements of energy, dimension, time and flame (or conflict) which he added to the reaver throughout defiance.