Some enemies are not melee-friendly.
After a few attempts with Placidusax, I finally defeated him with the Mimic and... a bow. I maxed out a simple greatbow and used Rain of Arrows (with expensive Radhan's Spear arrows). I was also wearing the two bow-related talismans. This makes for 2-3k per shot, and they're not too slow to execute. More importantly, it's possible to keep a good distance and to run away when he's doing his
laser show and AoE attacks.
The fun part was
finding this Ash of War. Originally, I saw an advice to use the Lion Greatbow, that I had, but the Greatbow just delivers more punch, so I checked if there was another skill than its default and found out about Rain of Arrows. When I finally got it, after falling to my death a few times, I saw one of the healthiest enemy out there: a mage golem. I couldn't defeat it - I don't even have any idea how far down his HP was, because for some reason, I can't see it. I wonder if maybe he's immortal? Apparently, it's also possible to get to this place by
abusing a spiritspring rather than going down the cliffs by jumping on roots and columns, which is very tricky.
I also had this Draconic Tree Sentinel bastard. Timing a roll with his lightning attack is very difficult, and hitting him is a risky business, but Hoarfrost Stomp got the better of him. I didn't touch him once with my blade, except to finish him off at the end.
Anyway, next on the list is Malenia.