Arno took the higher ground. He latched on the the branch above and looked down at his "prey". The snake was very fixated on Fuu, probably hungry for it's next meal. Assassinating it was going to be hard, at least outright.

So he had to weaken it. He had to think fast else this thing WILL eat SOMEONE, Arno wanted to actually avoid that, alright.

Arno knew what he had to do, and he made a dive right from snake to aim his blade into the snake's midsection.