Holy fuck is this place challenging.
Really illustrates why Owlcat is a bad developer.
So those Treants are CR 14. They have some pretty nasty attacks, but nothing that can't be handled with some planning and good builds. If the party reaches that location at level 10, that's an EL (Encounter level) +4 fight, which is supposed to be moderately challenging. But the player has Mythic levels, which balances things out.
But then the game throws two of them at you at once. When you add another monster of the same CR, that bumps up the EL by 2. An EL 16 fight at level 10 should only just barely be possible, and really should only be a boss encounter, not some random mobs on a map. But still, tough but doable fight, and Mythic levels again give you the edge.
BUT WAIT! They also throw a Spectre at you that is a
level seventeen druid (which means ninth level spells). The kind of magic that Gary Gygax once envisioned being limited to only unique high level undead horrors such as the Lich Vecna. You're really not supposed to give PC levels to random monsters, the rules weren't designed for it, except for extremely rare mobs (there are a bunch of these things on the map!)
First off, it's a CR 17 fight from the class levels alone, which puts it at the extreme high end of challenge for a level 10 party. On top of that, you have two nasty Treants joining in, which conservatively bump it up to EL 19. But the extra Spectre levels are the icing on the turd cake. Generally adding class levels to a monster supersedes its CR, with a CR bonus from the type of monster. A big giant with high strength might get CR +1 or 2, a Spectre with all the undead immunities and bonuses should be +3 or 4. I am comfortable calling this thing a CR 20 monster, fought with multiple nasty minions, making this a EL 22+ fight.
That's not even supposed to be possible to win. For a level 10 party, fighting mobs with such a power difference is a game master fail. E.g., a group of level 1 PCs can never beat a level 10 dragon in a straight fight, no matter how 'gud' they 'git.'
To quote from the DMG (pg. 38):
"The table does not support XP for encounters eight or more Challenge Ratings higher than the character's level. If the party is taking on challenges that far above their level, something strange is going on"
Something strange, indeed.
Before anyone fires off a smug reply, I did beat that fight, so shut up, but the point is it
shouldn't be. You obviously have to rest spam and buff like crazy to get through all of this stuff (and use Owlcat's extremely overpowered itemization), which is fine for some people but a playstyle I don't find particularly enjoyable. If Owlcat weren't interested in implementing actual Pathfinder/D&D, they should have used something else more suited to their goals.