I can't help you with Water MC specifically, I beat him using Lightning MC.
However, this applies to all mains:
-Try acupuncture, sleep, immobile (anything that stops attacks) on Sasori when he goes to use his mystery - though if you can't don't worry about it because it isn't as necessary as it was with Tobirama.
-Take out Sasori's puppets before Sasori. If you don't then he will just revive as one of his puppets. Clones that attack can be good for this (Sai is a good example)
-Other than that just high damage and combos
If it helps anyone I used Lightning MC, Neji, Kakashi and Karin.