Starcraft 2 Galaxy editor mirroring tools?
Solution 1:
Highlight the terrain, copy and paste it, but don't place the copied terrain yet.
Under the Edit menu, use the Flip and Rotate options to get it symmetric.
Solution 2:
Check
Map -> Symmetry
if you set it up before drawing on your map, all your changes to the terrain, decorations, units (e.g. minerals) will be automatically replicated on your map.