Well let me share with you what you skipped.
- Clam summoning sends out Mist made of Yin. Any opponent caught in its wide field range gets caught in its genjutsu. The only way to break the genjutsu is to break the Clam. The Clam is capable of camouflage and able to make false images of itself.
Now, one can argue Flower Tree World plus it to sleep, thus canceling the genjutsu. But with Onoki or Muu here to make it light, it is able to fly out of Hashirama range, the only method of Hashirama to beat the Clam. Flying invisable Clam, which can make false images of everyone + itself.
@Bold: Given that Hashirama's chances of landing a hit on the real guys is low already, Gengetsu weak body would be with the Clam hiding. Hashirama would be fighting Joki Boi, which Hozuki Clan nullifies physical attack too[
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