Boss Effect 1: Every 10 seconds or so the dragon breathes fire in a 180 degree cone in front of him dealing lots of initial damage and then damage over time for those standing on the flames for the next 4 seconds (= about 50% of the initial damage) and they 3 seconds after the flame breathe he tail swipes stunning for 2 seconds and doing about the same about of damage as the flame breathes initial damage and spins around to face that way.
Note: this mechanic happens a lot and I really like that you cant Completely dodge the effect like so many of your spells so its a nice change of pace. The player of course is supposed to go behind the dragon for the breathe and then move into the flames so avoid the swipe. Also since they will need a time or 2 to adjust the damage should be strong but not as fatal as some of your boss effects.
Boss Effect 2: Once in a while the boss will start rapidly shreding any target directly in front of it. For about 4-5 seconds each slash is a normal hit damage but he does about 4 per second or something like that. Player is supposed to move of course.
Boss Effect 3: At 50% health he triggers an additional flame breathe with no tail swipe.
Boss Effect 4: At 20% he enters berserking state and deals 3x all damage. At this phase I would also have an effect like inspiration that heals the players party to fullas they are inspired by the dragon's near death state to really keep it up.Give him a red glow for this phase.