Usually we use a hotplate or something that can heat the MCPCB from below. With hot-air/heat gun from the top it heats the LED for a long time before the solder reflows.
With a heat gun you can still heat it from below if you find a way to hold the MCPCB in the air.
Here’s a topic about reflowing LEDs with a video showing a couple of methods : How to reflow solder an LED emitter on a PCB or MCPCB.