There is a lot of adhesive on the back cover at the bottom of the S8, it goes about 1.5” up - its a lot of area to get through. Go very easy and make sure the adhesive is well softened with heat. You can go deep under the cover with whatever thin plastic tool you are using to separate the cover since theres nothing to damage in the area - but if you go too fast or pry too hard here the cover will break.

Aftermarket covers are very cheap (likely not gorilla glass), so it may just be better to be prepared to replace it if your original plan was to only replace the battery.