300C is really bad, you have probably melted or displaced internal components on the logic board of the phone.
I use my nails run around the screen.
I don't really pry the screen up but slowly loosen the adhesive by putting up the screen a tiny bit with razor blades.
80°C is what I use with a heat gun if I want to heat up the screen and loosen the adhesive.
75°C is what I use if I were to place the phone face down onto the hot plate.
I did manage to remove the Google Pixel XL screen that was working on the phone to replace the charging port.
1. Heat up around the edges of the screen.
2. Unfortunately you will have to use something like a razor blade to pry up the glass, but don't force the razor blade in, you just need to use it just to get your nails under the glass.
3. Once you got your nail(s) under the glass, keep heating it up and very slightly pry the screen up a bit and work along the screen adhesive around the screen.
3a. Alternatively you could just use a thin plastic tool to guide around the screen or playing cards instead.
3a. Note: Do not push too much into the screen on the edges, don't want to damage the LCD if it's not damaged.
4. It should loosen up enough after you do this a few times.