You can have some degree of RNG during a raid encounter, but not too much, otherwise it'd be a pain in the butt to balance. For instance, a boss alternating between 2 or 3 skills during certain phases. For instance: Thunderstruck in T9. Single/Double Charge in A10S. Optical Sight in A11S, with only 2 variations at first. Antilights from O4S, which may vary a little during GCO. Even if you know the mechanic, you'll have to pay attention to what's going on or you're ded.