This epoxy, commonly referred to as "underfill" is used to secure an IC on the logic board. Phones take a lot of abuse, even if we don't always notice it. You would never think of throwing around other electronic appliances like we do with our phones.
The trick to removing underfill is patience and the right amount of heat. Underfill gets quite soft at ~220C. So if you have a hot air station, set it at low flow and 220C and start heating the chip. Then use a sharp scalpel or knife (or probe) and start picking at it. you have to be careful not to scrape the logic board or lift other components.
That's it...sadly. There are no tricks or solvents that I know that will dissolve the underfill.