Apply a freshly heated iOpener to the back of the phone directly behind the battery for two minutes to help soften the adhesive securing the battery.

Re-heat and reapply the iOpener as necessary.

Alternatively, apply high-concentration (>90%) isopropyl alcohol under the edge of the battery to weaken the adhesive underneath. Let the phone sit for several minutes to allow the alcohol to penetrate and weaken the adhesive.