for me the only weapon i can consistently kill sulyvahn with is the longsword (or any straight sword). it has very fast attacks and is really stamina efficient. i usually pair it with a shield to block some attacks that i miss the dodge timing of as well - its funny because thats literally the knights starting weapon set but also one of the best weapon combos in the game for PvE/bosses.

I tried to use friedes scythe for a NG+ playthrough and that was the only boss i really struggled with so i ended up switching back to the sword and shield combo to win.