On the faceplate end of the battery, locate the seam between the two plastic halves. Slowly work a sharp knife between the two halves exactly as shown here.

The plastic tab that projects here has not been ultrasonically bonded like the rest of the case. You're using this to gain leverage on the parts that are bonded.

Don't let the knife slip and injure yourself. Plan your actions so that if the knife slips, it will move away from your other hand and the rest of your body.

You're trying to open up a gap between the two halves so that you can fit larger tools in between.