Well - I clean a 2 group an awful lot :(
It is an e61 group head - so you can find lots of information about it... But what I do is use a teaspoon and lever the showerscreen out. There is a tiny ridge near the bottom of the screen. The spoon gains purchase on that, and you lever against the grouphead. It moves only a tiny bit (especially if the seal is hard) - so you need to reposition the spoon opposite the original point and repeat.
The seal and the showerscreen come out together. It is a pretty simple job. For me - once the thing gets hard to remove I replace the seals. The whole removal thing takes under a minute for me now (thats 2 x groups!).
Hope this helps.