I too managed with just heat (iOpener that had come with parts for another repair) and the plastic card. I decided to try with just heat because of concern about putting any fluid into the computer case. (There was a comment by someone about damaging a trackpad). Using the card and heat, I found it helped to start by inserting and advancing a pointed end of the card rather than an entire long-edge of the card. Then advancing the card, while also moving it in a back-and-forth motion, to essentially “cut through” the glue strips. One other point, that helped for later positioning the new battery so the the speakers fit without issue, was that with a sharp pencil I traced the outer edge of the old battery on the inside of the case. This left a nice aid for worry-free placement of the new battery.

Pay careful attention to the orientation of this plate so that when it comes to re-installation you put it back the correct way. Sounds obvious but I’ve found it inverted in one customer’s phone, which led symptoms of charging failures and repeated spontaneous restarts.