How can I open the 2011 MacBook Air without damaging the screws?
You cannot safely remove it without the proper tool and the correct touch. You want to place the driver on the tangent to the case (the screws enter at an angle - not vertical compared to the keyboard plane.

iFixit.com and other places sell the pentalobe tool for less than $15 and it's money well spent to avoid needing to get Apple to replace the screws should you strip them.