There is a special screw driver designed by Apple to open the MacBook Air from 2011/2012. It's called a 5-Point Pentalobe Screwdriver. It's in the shape of a star. I had a client that had a liquid damaged MacBook pro and I needed to open the bottom quick.
A tiny flat head will strip the screws without fail! Since I didn't have the screwdriver on hand, I was able to borrow it from an associate technician who works for an ASP. I was able to salvage the board and remove the liquid from the interior before any major damage settled in. Needless to say, if my friend didn't come through I wouldn't have been able to save this board in time. So, don't let this happen to you. Order this screwdriver in order to prevent a corroded logic board in MacBook Air.
Trust me, no local hardware store has this screwdriver. If you don't know a tech that works for Apple and will lend you his tools - buy this one!