This achievement is easy if you have a powerful enough beam weapon to kill any unshielded ship (e.g. automated scouts).
This is an example of killing 8 hp AI ship without shields with Halberd Beam on sector 3. Cloak is lvl2.
1. Don't do any actions before your beam is almost ready (1-2 seс). You may suffer damage yourself.
2. Cloak and fire beam, damaging 4 rooms for 2 and killing the ship.
3. Beams do not interrupt the cloak, so wait until destruction animation is over and you get dialog box and achievement
Same may work if you fire your lasers or rockets just before you cloak, hide immediately once projectiles left the weapon and then during cloak finish the enemy with a beam. Your cloak has to remain until the victory dialog appears.