How to solder large smd components on a PCB with 2oz copper
I've had this problem before, the only way I've solved it is to heat the whole underside of the board with an IR reflow station for BGA's or using a hot plate.

The underside of the board needs to be substantially warm because most of the heat is wicking out through the internal planes and to the air. 2oz copper doesn't help, it's 4x better at transferring heat than 0.5oz. 

The PCB doesn't need to be very hot, maybe in the 100C to 150C range.

You can then use two soldering irons to heat the pads or a hot air gun to get the part on.