The dragon age wiki has plenty information about all enemies weakness and how to defeat the stronger bosses.
This is a god of night
, so I think frost and spirit are his main attack and fire and nature his main weakness (those are also the weakness of the archdemon in DAO).
If you kill 5 enemies and left 5 with full health you take less damage in the end than if you have 10 damage enemies doing full damage until they die.
Fire and thunder area damage are not very effective against rock wraiths.
Scattering your long range units is good to avoid area damage, while having the close range units near each other is good for benefical area of effect.
Researching as the best solution is a lesson you should take to your life and never forget.
There is no shame in not knowing and seeking help, but I agree it's a lot more fun to find out strategies by yourself (yet a lot less effective).
And congratulations to those who are able to win at hard mode!
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Next comic: Retrospective of Act4
Is Act4 almost over? Yes, but I plan to do Act5 eventually.