I have replaced 3 batteries in rMBP (2x 2012, 1x2013) and done all sorts of work on our iMacs (2011) including graphics card repair. First time you do any of these is with trembling fingers and it takes a long time. But that changes quickly with the 2nd, 3rd etc. repair.
The batttery replacement for the rMBP is made difficult because of these being glued in. The recommended manner to remove the batteries is by using a desolver fluid - iFixit provides this including goggles and gloves. This fluid needs to be dripped and drizzeled just in the area you are working in and you should not let it spread. I did not like this, too easy to make a big mess. Instead I (strongly) recommend to “saw” the batteries out by means of a strong cord you slip underneath.
You would need a strong, thin cord (home depot, marker line will do fine). Cut a 2-3 foot piece hold the ends with both hands, slip the middle part under one of the battery sections and start moving it back and forth while applying some pressure. I found the easiest to do this is by pulling the cord towards me while making the sawing motion. You may use gloves because it will start to cut into your hands.
I can assure you, this sounds a lot more difficult then it really is. I had my first battery out in 30 minutes, no fluids nor goggles needed. The iFixit batteries I use are very good and with proper charge/discharge management now over a year later show close to original charge and some even better.
But if it does sound difficult, I would not waste time on any other method/site and just go to Apple with it. More money but their service is outstanding.