When attacking while in Cloak mode, you automatically lose all your energy. Instead, go to Armor mode (not Strength mode as this takes up energy when you attack), immediately shoot a soldier in the head, then go back to Cloak mode. They will not see you and you will still have all your energy (and even may gain 1 to 3 %). While you are transitioning to Armor mode, you will have a couple seconds where you are still invisible to the enemy, which allows you to adjust if you are sniping. Even if you reach your time limit that you are invisible after turning off Cloak mode, you can still shoot around and might get away with not being seen as they occasionally panic and move around too much to notice you. You can use this to stay in a stationary location that the A.I. does not touch and easily take out a huge group, (such as the area directly below a watch tower).