Energy crystals in "Final Fantasy VIII" serve a variety of purposes. Using them on your Guardian Forces will teach them the SumMag+30% ability, which increases their summon damage. Energy crystals are also an ingredient in creating powerful weapons for your characters. To easily obtain energy crystals, you can either win them from enemy battles or create them by modifying cards.
The powerful Elnoyles drop energy crystals at Level 30 and above. An easy way to find an Elnoyle is to take the Ragnarok to Esther during Disc 4. Once you've landed, guide your characters down from the air station until you reach a fork in the road. Go down again and you'll arrive at an area with a moving platform. Follow the flashing arrows until you come to a set of stairs. Down the steps, there's a man who will turn into an Elnoyle if you speak to him. Win energy crystals by defeating the Elnoyle, then run off-screen and return. The man will be there again, and you can repeat the process as many times as you like.
Behemoths and Ruby Dragons
Two other enemies that routinely drop energy crystals after battles at higher levels are the Ruby Dragon and Behemoth. Both enemies will present a challenging fight. Ruby Dragons appear as random encounters on The Island Closest to Hell. It's the westernmost island on the world map, accessible only with the Ragnarok. You can find Behemoths in Esthar and the surrounding plains, and also on the Lunatic Pandora.
Although it's a little more time-consuming, you can modify cards to get an energy crystal without going into battle for it. After the Guardian Force Quezacotl learns the Card Mod ability, you can turn your Triple Triad cards into useful items. Ten Elnoyle cards will modify into one energy crystal. Collect Elnoyle cards by defeating Elnoyles in battle, or win them from other card players throughout the game.
To increase your chances of acquiring an energy crystal during battle with the Elnoyle, Behemoth and Ruby Dragon, use certain abilities. The Guardian Force Tonberry has the LV Up ability, which will enable you to raise the level of the enemy you're fighting. An enemy with a higher level will be more difficult to defeat, but will also have a higher chance of dropping rare items like energy crystals. Additionally, you can use the Guardian Force Bahamut's Rare Item ability, which will increase the chances of rare drops as well.