This is a very easy achievement.
Just start the level "Troll Warehouse" with your favorite Skylander.
You will come to a minefield, for that you will need a map to cross it, you can't cross it unless you have that map.
Proceed the level to collect the four map pieces.
When you have the map, a white dotted line appears, just stick to that line.
The mines will rise from the ground and start to blink red and make some beep sound whe you get too near.
So stay away and move throug the field when you have the map.
The Minefield ist also the only place in the level where landmines appear.