exactly where am i supposed to place his special terrain?

Please quote the game effect in question.

Planetary Destruction: When you reveal your force, you may
place 3 "DRILL" special terrain markers on the map outside of
starting areas. At the beginning of your turn, as a free action you
may deal 1 penetrating damage to each character occupying a
square containing one of these markers. At the end of your turn, as
a free action you may place each of these markers into a square
adjacent to its current square that doesn't contain another
"DRILL" special terrain marker. When Imperiex is KO'd, remove
these markers from the map.


And that's as complicated as it gets. Due to the Golden Rule of Occupancy, you can't place them in blocking terrain. Otherwise... anywhere but a starting area.