You gain Energon points for successful attacks and conserving AP between rounds. As you progress through the game you'll also encounter zones where placing squad members also charges your Energon bar. Once 50% full you can perform an Ultimate attack. 100% full Energon bar allows for more powerful versions of these moves. There's no real need to go hunting for excuses to use Ultimates as you'll get plenty of opportunity. You'll very likely get this achievement simply through the normal course of play. Note that you can get two ultimate attacks (albeit of lower intensity) from a full bar. The Ultimates also work with characters who have exhausted their AP for that round (effectively providing an extra go) as well as still functioning for characters who are suffering status effects such as anchoring.