No better way to use a wall mine than to catch the dirty punk who wants to kick the door in on you! Here's how to do it: Plant the mine RIGHT ON THE DOOR that you are sure an enemy is on the other side of. Then shoot your gun at the WALL that the door is on, or make some kind of noise that the guards can't refuse to check out. Then you step VERY FAR AWAY (but still in visible range so you can watch the fun) And see the guards get blown away trying to blow YOU away when the kick in the door. Never gets old:).
Kill Two Birds With One Shot
First of all to kill two enemies at once, you need the SNIPER attachment. Engage the sniper attachment (once equipped you need to zoom with R3 and then hit the L1 button to shoot the sniper rifle) To get TWO enemies at once, you either need a situation where both are very close together, either talking or on alert, and then fire through the first mercenary's head and if shot well, the bullet will carry through the second one's head before he can react.
EXAMPLE: On the last mission, before you encounter Emile on the rooftop, two guards are in a wide room (with a turret and trees on both sides) You enter the room silently while both are patrolling away from you, on the right of the door is a bottle. Throw the bottle far away to the left, in the trees beyond the guards. Get out your sniper rifle and watch them both check out the noise. They'll tell each other to forget it happened, and when they are together or moving back, take them out. If one is in front of the other, a shot through the head will take care of them. Saves ammo huh?.