I beat the rotted greatwood after Sunbroing it a few times for other people to learn its moveset.
He has a lot of polyps and eggs and stuff on various parts of its body in second phase, so it's a lot of running and rolling around to get to those points. He exposes different polyps with certain moves, like when he falls over, so you can reach some that you can't otherwise. Hitting any polyp enough times makes it explode for high damage on the boss. Sometimes you can sneak in a few hits on the hand too.
A weapon with a vertical moveset seems to really help reach some of the polyps on this boss. I two-handed the regular longsword since the R1s in that moveset slash up and down.
Speaking of the longsword, I'm surprised at how much I like it. Its moveset is just so versatile, and the 1h R2 stab has a level of range and damage that I usually associate with spears and polearms. I just got a boss soul greatsword and I'm leveling up so that I can use it, but right now I'm very happy with just the longsword.