I'm not sure about this, but I think they have lots of patterns. But some of the actions in a turn may be random.
For example : During the final boss fight, I've written the pattern of the boss. I died and I retried. Then, the boss used the SAME type of pattern in the SAME order, however one action changed per pattern...
Maybe it was just luck, because other boss didn't use the same order for pattern. But I think they definitely have LOTS of pattern and one or two actions during one turn should be random (ex : A B B X X G ==> X could be either A B G or skill)
"Power makes you mistake Truth and Truth makes you wish for Power. This Foolishness is testimony for you, Human" ~ Arcueid-sama
some bosses tend to use their skills in their second action or their last one.
Assassin also always started using a buff skill in his first action (and sometimes using next his NP) their normal actions are hard to predict, but their use of skills are always in the same positions, so being in the safe side and using block in those positions is not a bad idea.