When you attack while in Cloak mode, you instantly lose all your energy. 
Instead, go to Armor mode (not Strength mode as this takes up energy when 
you attack), quickly 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 few 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 sometimes 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 large group, 
(such as the area directly below a watch tower).