Prepare an iOpener and apply it to the screen opposite where the battery sits. The heat will help soften the adhesive holding the battery down. Do not apply the iOpener or any other form of heat directly to the battery.
Apply some high-concentration isopropyl alcohol or adhesive remover along the bottom of the battery and then tilt the phone so the liquid will run along the bottom of the battery and weaken the adhesive. Then wait a few minutes and try removal again.
Carefully slide the flat end of a spudger under the battery to slice through the adhesive and lever the battery out of the phone.