Fracture

There is a section in Act 2 where Jet will exclaim: "They've got a lot of Lodestone cannons around here" or something of the like. Kill the enemies on the platform and grab the cannon.

Down below, there is a contained area with two Creepers. Shoot a shell to the edge of the platform that you're on to suck all of the boxes that are on your platform down into that contained area.

With these boxes down there, shoot a Lodestone cannon shell into the middle of the boxes so that both a Creeper and the boxes converge to the shell. This will kill the crush the Creeper to death. After its death, another will come to take its place (there are always two in the area) and will continue to respawn. Also continually respawning is the ammo for the cannon. Continue to crush the Creepers in this manner until you get the achievement.

Note: If the boxes don't crush the Creeper, shoot a shell to move the boxes away from each other so that when they do converge on the shell, there is some speed to the boxes.

You can earn this achievement in "Weapons Training" mode, which unlocks after you have found 10 data cells. Equip the weapon, then stand in one of the red, pulsating circles to summon enemies. There is a red circle downhill from the building you spawn in, and another on top of the tower to the right.
Some of the weapons don't unlock until you have found a set number of data cells, but this particular weapon is available either as soon as you have unlocked Weapons Training mode, or at worst when you have found less than 30 data cells.

In the last section of Act 1 you will have to take four scramblers off the Golden Gate Bridge. This is where you will first encounter the Lodestone. There are two crates containing a Lodestone, one above the two turrets and another on a building located to the right/rear of the turrets. Complete this section and make sure you take a fully loaded Lodestone with you.

The next section you are in a cave so just follow the path and you will come across enemies. Fire your rounds into the walls directly above the lava and it will suck the enemies towards it killing them. Continue and you will see even more enemies. Fire FOUR rounds directly into the ground behind the enemies, it should cause some boulders to come down and smash all of them in the vortex. Reload checkpoint and continue until achievement unlocks.