Buck Converter 24V 12A
Add this to your gate drive: 







simulate this circuit &ndash; Schematic created using CircuitLab

You need to charge and discharge the gate fast to minimize switching losses. You can also buy a real gate driver that can source or sink amperes of current quickly. 

Also, the IRF530 is a bit wimpy for this application. Try getting one with about 1/10 the Rds(on). Remember Rds(on) increases significantly with temperature. For that reason you generally don't want to thermally couple paralleled MOSFETs, but you're better off just getting a better, more modern, part.