Please be aware that this emulator does seem to be laggy at times.
To get achievements working on the GameBoy Advance core system. You need to install an emulator called MGBA.
You will want to Git Clone mgba into the root directary.
git clone https://github.com/libretro/mgba
Next go ahead and compile the project:
Go to the mgba folder and run the make file
cd mgba make
this will take a while
Loaded SSH on your PC and connected to your Pi.
Now you will want to edit
You will want to add this to line 3
mgba="/opt/retropie/emulators/retroarch/bin/retroarch -L /home/pi/mgba/mgba_libretro.so --config /opt/retropie/configs/gba/retroarch.cfg %ROM%"
You will also want to move the default to line 4
save and upload.
Now reload retropie and enjoy GameBoy Advance with achievements.