Hailscorn (Tactics) Edit
Hailscorn is a skeletal dragon. It has an ability to summon lots of skeletal mob adds, and if they are left alone they hit for enough and fast enough that if they aren't destroyed, then your life total starts going down uncomfortably fast (especially since the dragon is still there pounding on you). It also has a big knockback that can send you flying down the steep mountainside. Fortunately, the Etymidian does not seem to take falling damage, though other mobs you land in will agro and run over. This can include Urgreth of the Thousand Tombs, who is further down the mountain slope and off to one side. I found attacking Hailscorn from higher ground to be convenient to avoid the knockback (or at least I did not seem to get knocked anywhere when I was on higher ground).
You run over and engage it in combat and use ability #1 (cleave) until it summons skeletons and then you use ability #2 (AoE) to destroy the adds. It is tempting to clear the pre-existing wandering mobs using ability #2 at the start of combat. I recommend waiting until the adds summoned. Ability #2 uses 5,000 mana and you only have so many uses. Ability #3, the heal, takes 10,000 mana, and using it may make it so that you do not have enough mana to destroy a summoned group of skeletal adds. Using ability #4 (mana regeneration) did not seem to be possible in this battle, the dragon was usually right there pounding on you (ability #4 is interrupted by damage, it appears).
Note: Using Ability #5 stops the battle, removes you from combat, and resets your target.