Nah, Alissa's a bitch (to fight, too), don't get me wrong, but even on dark difficulty, as long as you're at least lvl 5/6 (which you probably will be unless you just mainline your way to the Winter City ASAP), she's perfectly beatable without too much trouble, as long as you pick the right companions/powers.
Now, the one enemy/fight in this game which, IMO, has absolutely no right being as retardedly difficult as it is, is Ramien from the second part of "Over Troubled Waters" (Varena's quest). The frustrating part with him being that he's not even necessary THAT tough of an enemy by boss standards, but that the difficulty of that fight is a simple consequence of (IMO) nothing more than bad game-design. Namely the fact that you're saddled with a pair of AI-controlled teammates, and no matter how much I might love Livy or Varena, the allied AI in this game kinda sucks.
If you could only bring your normal party members, Ramien probably wouldn't be that difficult at all. But because your teammates are AI (and the AI, again, sucks) the Champ is essentially forced to perform ALL combat roles simultaneously, and pretty much by themselves.
Healing? All on you. Because even if Livy/Varena have healing spells (which I think at least one of them does?), you can't control when they use them or on who, so good luck from a survivability standpoint otherwise.
Damage? You again, because while they have some good spells, the inability to coordinate blows so as to take the bastard out quick makes them nearly worthless, and certainly unreliable. All of which is exasperated by the fact that Ramien heals absurd amounts every turn in phase 2, practically requiring serious alpha-strike capability to take him down... which requires coordinating power usage, which possibility, again, the AI teammates deny to you.
Tanking? Literally mandatory, because Livy and Varena seemingly have no defenses whatsoever, and if he ever targets them Ramien will knock out one--or even both---of them in a single turn, and possibly even a single hit. But of course Champ-tanking is objectively the worst and hardest way to play the game, so forcing that makes an already awkward fight even more artificially difficult. especially since the bastard gets to attack like 3/4 times per turn for absurd amounts of damage. so you can't even rely on something like the Dawnsword's Deflection to keep you safe.
The end result of all this being that the fight, moreso than any other fight in the game (for reference, I've smoothly first-time cleared the Coastlands rock-golem on multiple instances, on Dark), frustratingly feels almost purely RNG/slot-machine-esque; like the outcome is more a matter of random chance than any particular tactics or abilities on our part (which, because of the AI teammates, in many ways it quite literally is).
