I broke a case opening tool, that looks very similar to the blue one shown, at the first attempt. Since then, I've successfully used a set of guitar picks of different thicknesses

Note that it is very easy to break a screw holding the battery connector - don't over tighten. I've managed to hold down the end of the connector, towards the center of the phone, with a washer fitted to a nearby screw. It's been working fine for 6 months