I heated the back of the phone on a heating pad set on high, but both adhesive strips still broke almost immediately. Tried a credit card but that got nowhere, so then I remembered seeing a video in which someone used monofilament fishing line. Worked perfectly. Just pulled it behind the battery, pulling on one side then the other until I worked it nearly all the way down, then the battery just lifted out. Wrap it around a couple of things to use as handles when you pull and it’s less painful.
I liked the idea of using an iSclack to pull on both sides at the same time but didn’t want to spend the money for one. Instead, I used the suction cup that attaches my GPS to the windshield of my car, attached it to the front and then attached the small suction cup provided with the tool kit to the back of my iPhone. Pulling both in opposite directions worked perfectly to pull the case open with minimal effort and no surprises.