This achievement can only be earned in the Strategy Game, not the Campaign.
Start a strategy game with the following settings: Captured Islands Count (Half), Enemy Strength (Midway between Medium and Strong) Startup Resources (High), Islands Output Rate (High). You will start with three islands. Provision up as desired and start going to small islands on the map (while the island placements are random, the individual islands themselves are exactly the same layouts as in the campaign). When you find one that is listed as Strong (press on the strategic map to see the rating), capture it. Strong islands are actually about a medium difficulty in the game, but save frequently to avoid issues. When hacking the command center, save just before completing the hack. Reload that save 49 times and you will have all three of the "Iron Fist" achievements.
Note that this achievement does not stack with the various "Bunker Buster" or "Balls of Steel" achievements.