While Pulling on the suction cup, slide a thin plastic spacer (guitar picks work well) between the back cover and the phone. Slide it around the edges to break the adhesive seal. Be careful, but don't be afraid to use some force. You should hear a sticky breaking sound.
Remove copper plated tape and any other securing adhesive with a nylon spudger. Remove carefully, as it has the potential or tear if too much force is used.
Pry the battery from the phone with the nylon spudger. There is a small amount of adhesive underneath the battery, so a bit of force is required. The battery is malleable, so it is possible to dent the battery if too much force is applied in one spot.