Greed Corp
12 Achievements
200
With Cannon, you can
You fired a cannon for free at the one who lost it to you
15
How to unlock the With Cannon, you can achievement in Greed Corp - Definitive Guide
During the Cartel campaign (that's the third one), simply send a walker to capture an enemy cannon (must have at least 1 bullet preloaded), wait until your next turn and fire it at the faction you captured it from.
I feel the last map in the Cartel Campaign - Evander's Cliff - is easiest for this achievement. Its one on one and the CPU will almost always build a cannon around turn 5.
This must be done in the Cartel campaign, which takes place between levels 13-18.
First, allow the enemy to build a cannon and load it. You will know when it is loaded, as there will be a number on the tile next to the cannon, denoting how many shells it has. Move your walkers onto the tile to capture the cannon. You need to defend the tile, as you cannot fire the cannon until the next turn. Thus, I suggest you have a second group of walkers, ready to advance on the tile to protect it.
When the next turn comes around, select the cannon by pushing on the tile it resides on. Aim at an enemy tile, then press again to fire. Achievement unlocked.
Note: Whilst this can be done on any level, level 18 (Evander's Cliff) is probably the best to choose. The computer builds a cannon very early and usually leaves it relatively undefended.