The Tesa tape is to adhere the battery to the frame. You don't want anything moving around inside your phone--that can stress, damage, or disconnect ribbon connectors.

The iOpener doesn't open a device. It's just a big radiator that's useful for keeping the temperature up. An alternative is repeatedly heating with a heat gun, and allowing parts to cool down while you cut through the gummy goo.

You cannot heat the adhesive such that the pieces can simply be pulled apart. The adhesive should be heated so that it becomes soft enough to cut through with a plastic sheet or playing card. I heat an edge with a heat gun, keeping track of the temp with a non-contact thermometer. When a side is up to temp, I then use the iOpener to keep it from cooling down quickly.

Take the time to read through the processes. Watch a video or two. Stop blaming the tools.