Zabuza's Sword - Pretty Lame.

[BenjerminGaye]

sasuke cut off a zetsu head without proper stance and one-handed without the use of his lightning chakra

zetsu have also been shown to be quite durable, more so than a regular human being.

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What? no he isnt...is he? i mean obito ripped off his zetsu arm quite easily...Minato rasenganed obito's arm and it fell apart...Mei blew spit hot steam at it and it turned to jelly while sasuke who was in the same room got small burns...Raikage killed one quite easily...I'm not sure about that.

Besides that was a surgical cut.(like when doctors pull your skin apart as they cut) Since the zetsu was being held up by his head as sasuke cut it the weight of his body fell to the floor instead of collapsing on the blade. Essentially by holding the zetsu in this manner your're reliving tension off the blade making the cut all the more easier.