Because of the power level of these devices, it is recommended to use a big unbroken single ground plane for the whole system / board. The ground plane can be easily made at bottom PCB layer. In order to minimize the impedance and inductance of ground traces, the traces from ground pins should keep as short and wide as possible before connected to bottom ground plane through vias. Multiple vias are suggested to reduce the impedance of vias. Try to clear the space around the device as much as possible especially at bottom PCB side to improve the heat spreading.