The Ballistazooka can be found inside the Sentry's Grave dungeon in Mortal Shell 2. To get there, you need to explore the southern Mammon region until you reach the Gate of Mammon Beacon. From there, head up the Lonesome Spire and look for the dungeon's entrance along the main path. The weapon is dropped as a reward for beating the Subjugated Guardian, one of the more frustrating of minibosses in Mortal Shell 2, so be prepared for a fight.
