Random Skills are abilities only found in high-level bosses. For instance, the Zombie King on the 41st Level doesn't have any Random Skills. That one is the weakest of the Zombie Kings, with all Basic Skills, and you figure out the tactics after one go. As you climb the levels, the Zombie Kings get increasingly stronger. Besides the enhancement of their basic Attributes, the main difference is in their Random Skills. They randomly deploy a few from hundreds of potential skills, making them unpredictable! Now we're on the 50th Level, where the Zombie Kings are at their strongest. Their Random Skills come as naturally as breathing, with no discernible pattern.
Due to the existence of Random Skills, the effectiveness of guidebooks is greatly diminished. Even the strategies from top teams can't cover every Random Skill, so the ability to improvise becomes especially crucial.