if your servants attack a certain number of times their skill will fill up and they will use it. The more stars the servants have the shorter they need to trigger their skill. But it can be a bit tricky if you kill the enemy too fast. You might use weaker servants to delay the fight and trigger their skills. If you managed to make your servants use their skill 5 times (e.g. every of your 5 servants use their skill once) you will get the cristals.