I'd level to 11 at a minimum, personally. That's when you get your last class ability, and many of those are fantastic (you're a ranger - stunning shots is fantastic, although less against Thaos than everyone else since he seems to still move when afflicted by it. It'll still tank his defence).
Again, you're a ranger, so buff your accuracy and use Binding Roots to make sure you're the only person in that room who gets to stand where they want to.
Disorienting weapons will make Thaos die much, much faster - you may have a bow called Lenas Er (optional quest reward) which will really ruin his day. There's several other weapons sold by merchants that apply that debuff - I quite like the hammer Strike Hard from Hearthsong market.
Make sure you have all the figurines, and there's always a couple of summons on the field - they can draw fire. Basically every figurine is better than Kana's skeletons, and you said you're using them a lot.
Play on slow-motion. If Thaos starts casting, move everyone who is standing still somewhere near where he's casting. By the time you get through Sun in Shadow, you might have a couple of pairs of Boots of Speed - they can help you dodge stuff.