This challenge sometimes activates when Boom is killed before Bewm.
The easiest way to get this challenge is to cross the bridge and stand at the edge of the lift chute to alert Boom and Bewm to the vault hunter's presence, then to run back through the small camp up the ramp and hide behind the hut. Big Bertha's long reload delay leaves Boom open to sniper headshots. Once Big Bertha is disabled, Boom and Bewm are fair game. Boom tends to mill around Big Bertha, making him a fairly easy sniping target, but Bewm will more actively attempt to follow the vault hunter and as such will often be just out of sight behind the metal fence surrounding the area.
Another way is to run past the turret on the left side and shoot it repeatedly with an explosive gun. Because of the slow rotation speed, Big Bertha can be destroyed before it can aim at the Vault Hunters. Rocket launchers can disable Big Bertha in minimal number of hits.