Answer:
A man shot two arrows at its center and missed both times, but using a sword swung once at its center and hit it twice.
We are all around, yet to us you are half blind. Sunlight makes us invisible, and difficult to find.
Turns us on our backs, and open up our stomachs. You will be the wisest of men though at start a lummox.
The more of them you take, the more you leave behind. What are they?
I tremble at each breath of air, and yet can heaviest burdens bear.
You use it between your head and your toes, the more it works the thinner it grows.