In general use either an expendable or hard to hit or very "tough" unit to draw fire by moving it in small increments across an area in line-of-sight/line-of-fire of the enemy, while your other units are in overwatch positions to see what fired at it...preferably where only a small portion of your opponent can get in a shot.
The supress or kill what exposes itself.
Rinse.
Repete as necessary.
Eventually you win.
