This Is Done On The Third Map
Build A "Siege Breaker" And Push Up To A Portal To Start Damaging It.
Be Careful! Upon First Approach, Most Greed Portals Seem To Spawn Slug Beasts To Halt Your Advance.
Slug Beasts
- You Can Backpedal Your Siege Breaker Away From Them
- Keep Them In Range Of Your Backpedaling Archers!
- Killing Them, Then Pushing The Siegebreaker Back Up To The Portal Seems To Not Spawn Another For A While. (Spawns Might Be Based On Total Portal Health, Not Sure Yet ;D)
Destroying A Portal On The Third Level Nets You This Achievement.
Good Luck!