Max pushed his unit forward toward the enemy as he unleashed the Disintegrator on a wide dispersion pattern. Now that he knew the vehicles weren't properly radiation shielded, there was no reason to put all his power reserves into attacking, the pilots would die either way and disable the vehicles.
The aliens cursed the Mecha squad as it advanced, and the vehicles died an instant before the bombardment started, leaving them confused and trapped within the destruction.
None of them seemed to understand that the weapon Max was using was not visible to their eyes, or to most sensors, and for the first few seconds, confusion reigned.
Then the officers managed to get things back under control, and the survivors took positions either under the disabled vehicles or in craters to begin returning fire.