To remove the battery, you will need something to warm up the back of the X7ii. Also ideally a long, very slim and strong plastic putty knife or spatula. There is an adhesive pad keeping it stuck to the back of the case.
If memory serves me (it was a long time ago now, check my past posts in this thread) I was able to open it up and remove the battery after removing the screen and main board (which stay together). The biggest trick (after removing the two screws holding the bottom of the screen, visible after removing the amp section) was figuring out that there are 3 clips on each side of the screen that clip into detents in the side of the case. I believe you also need to pry off the thin metal panel stuck to the top quarter of the back of the case, to remove a small WiFi board (antenna?) so that doesn't hang you up - at least I did this first, as I was trying to figure it all out...
Hopefully the photos I attach will be visible and give you a clearer idea...
I had the same issue recently described here (see my earlier posts). Tried buying a replacement X5iii replacement battery, removing both Li cells from their attached small circuit boards and soldering the new cell to the old x7ii board - unfortunately after doing that, that board was not supplying any voltage at all on its outputs. Not sure what went wrong. I should probably dig it out again and quadruple-check in case I just had a bad solder joint that I didn't catch earlier...