Ordered the battery from iFixit, forgot to order the Pentalobe screwdriver. Nevermind, if you only do that stuff once (as I expect after 6 years with one battery) - don't spend the money on special equipment that cannot be used for anything else :)
Opening the macbook is easy (I have done that before to exchange the superdrive with a larger harddisk and the original HD with an SSD drive, and a couple of times to clean the fans), but the 3 screws on the battery are a pain.
What did the job for me: an old mini-flathead-screwdriver (approx. 2mm? - just slightly larger than the pentalobe) and a file. As none of my other screwdrivers would work with the ....pentalobes, i filed down the flathead (-)screwdriver so that it could get hold in 2 of the lobes - which did let me open and close these screws without a problem. My screws were not closed very tight though - if they are there is probably no way around the right tool.