The only way I have found to build up the power bar far enough in one round of multiplayer to get to the hero vehicle is to kill all 64 enemy soldiers manually through a turret and the enemy hero twice.
Obviously this is difficult to achieve in a ranked game so it is easiest boosted with a friend online or done in offline split screen co-op with two pads like I did.
I built one turret by the toy box then the hero needs to run over to the turret, die and as soon as it respawns it needs to run over again.
Once you get to the hero vehicle unlock then this won't deplete so getting the barrage in the next round is more straight forward.
The clip below shows me unlock both the vehicle and the barrage. The trophy pops when you select the vehicle or barrage to spawn.