The battery was having sudden drops (50%) in remaining battery life.
The adhesive strips hold the battery super tight and broke easily. Prying with credit cards wasn't getting the battery out of the case, and prying with a plastic spudger tool punctured the battery causing a small flash fire and "pop/explosion". I don't have a blow dryer so I triple ziplock bagged my iPhone and let it lay back-of-phone side down in nearly boiling water for 2 minutes. Battery came out pretty easily after that.
When reassembling the iPhone, the digitizer/touchscreen cable came disconnected while attaching other cables and I didn't notice. So after finishing the repair I had to backtrack all the reassembly steps to re-attached the digitizer cable.
Triple zip lock bagging the iPhone and laying the phone in nearly boiling water for 2 minutes seemed to work really well to loosen the adhesive holding the battery.
Don't use a credit card you are still using for financial purposes - card(s) get fairly bent while removing the battery.
Taping a loop of packing tape around the iPhone when opening it with about 1 inch of loose space in the loop around the fingerprint sensor end of the phone when opening it helps prevent suddenly opening the iPhone too wide if using suction cups and damaging the fingerprint sensor cable.
To keep screws in order of removal for reassembly I taped magnets from junk mail on a piece of paper and placed the screws on the magnets.
It gets hard to see inside the iPhone and see small parts, so I like wearing a headlamp while doing the repair so everything is very well lit.
When placing the new battery in the case I was very tempted to completely skip using the replacement adhesive strips because they we such a pain to remove. Considering however that a secure physical connection with the back of the case is probably important for heat dissipation, I took a half step - I only used 1 of the 2 adhesive strips so I still have a secure hold of the battery on the back of the case, to support heat disapation but also have the sides of the battery loose to hopefully make a future battery replacement easier to accomplish.