Is an attenuators power rating for average power or max power?
I would expect the power rating of an attenuator to be the average power since it is based on the heat dissipation capability of the device.  However, in your case, the peak power, 250 watts, is more than 30 times the average power (7 watts). Therefore, it is important to know the duration of the peak power pulse versus the thermal time constant of the attenuator. For example, if the peak power pulse lasts for 5 seconds, and the thermal time constant of the attenuator is only 1 second, then the attenuator will heat up substantially during the pulse and will probably melt, regardless of the duty cycle of the pulse. You should definitely carefully check the data sheet of the attenuator and/or contact the manufacturer to see if your application is suitable for the device. Just Google RF attenuators to get suitable manufacturers.