Berserker drops 4 rock piles on players, then immediately after places 4 stack markers. Like the only logical thing would be stack with those rock piles, since you do not want to multi-stack and there's nothing else that could probably save you.
Mortal Flame in Lugus and puddles in Nixie are not explained well, i agree, but both boil down to experimenting with mechanics and trying to see what works and what not.
Boss turning into a specific direction and starting casting something likely means you need to move behind it, also being turned into toad does not directly kill you since it does no damage and you cans till move while transformed, and you can clearly see the shape of the attack, so you won't (ideally) get hit the next time.
Death Ray is that way by design. You're supposed to get hit by it, and move out of the beam, it's why it does continuous, but not immediately lethal damage.
And if you see boss charging a massive laser and see that there are small separate spaces on the sides of the area, those are probably the safe spots to stand.
Like all in all a lot of those require you to just pay attention and learn, like all encounters in the game, really.