Difficulties getting back of case off phone

With the amount of Galaxy's I've done battery replacements in (My favorite's being the S7 and S8 series), I can suggest these steps.

1. Whilst the iOpener is great, don't get me wrong. I personally find using a hair dryer (On High Heat) or a Heat Gun always does the trick and in a lot less time. If you have access to either, heat up the Rear Glass for around 45s or 1 min.
2. After that, attempt to get a guitar pick (Or other slim durable material) in-between the adhesive and glass. I find around the sides of the S7/S7 Edge are the easiest to get my pick into. Hopefully, the original 1 minute of heat should be enough to pop it off. If you're still finding it difficult to find an area of access, you should go back and heat up the glass for slightly longer.
3. Congrats, it's off. Have a break, and take some time to clean up the glass (If shattered in the process).

Remember, you will need to clean off the old adhesive on the mid-frame (and rear glass if you're reusing it) to ensure a perfect fit when you go to put it back together!