Removing QFN Lead Frame From PCB
You'll need a h*ll of a lot of heat to get that solder to melt, that's by design, all those vias and probably 2oz or greater copper thickness is designed to spread the heat. 

An air gun probably isn't going to be enough. There are specialized rework tools, but for a few you might want to try a BIG soldering iron, say in the 120W range, after preheating it to 100'C.