So, if your group didn't manage to kill the serpent, would you still be able to earn rewards/fragments?

It's exactly like Ninetails event. The snake is immune to debuffs/displacement, weak to elemental damage...

But unlike the fox, you can go a full 10-rounds with it.

So have fun! It gives some nice coins and experience. :))

Well, the snake does relatively low damage, but since it attacks repeatedly, the damages stack. If you are dying too fast, then i would say bring a healer. However, the goal is to cause as much damage as possible to the summon while staying alive; there for it's better if you bring an offensive healer (Karin, Tsunade or Sakura) for the extra damage. The snake doesn't cause any status debuffs, so you can bring Guy to help your team in healing.