My first game with the Dreamer was also my least satisfying game of all time; I won, but only in a very cheap and silly way. I later managed to beat her more honestly, but today I succeeded in losing to her, as I should have in that first game.
I feel like it's thematically fitting to always put Dreamy in a particularly dangerous and terrifying environment; why else would her mind be lashing out? (My first fight with her broke this rule by taking place in The Block, which is about the most tranquil environment there is, and she shredded my party without any help from the Inmates.) The better of the two previous games was in the Final Wasteland, since that's about the worst place imaginable, and Our Heroes succeeded in rescuing her from it; for this one, I went to Insula Primalis, figuring "hey, what kid wouldn't want to see dinosaurs", and was utterly crushed. Dreamy opened on a mean bunch including Illusory Demon, Macabre Spectre and Grotesque Arachnoid, then fired off Night Terrors on turn 1; Expat skipped her first turn due to lack of Guns, AZ hurt himself with fire damage since he lacked his healing Module, and Visionary drew Brain Burn and Demoralization which were both dead cards in this match; only Tempest was really strong. This was the first time I've seen the Velociraptor Packs attack a hero; AZ died to Granite Oni on villain turn 3 after Enraged T-Rex and the Raptors had shredded him down to 1 (he went from second-highest HP to lowest with one T-Rex chomp); the next round, Primordial Plant Life took away a 5-HP Expat's Flack Jacket before it could deflect the T-Rex, who instead ate Tempest since he was down to 3 after happily destroying his Localized Hurricane. During the intervening round, Tempest had recieved Incap AZ's damage boost and used the Hurricane to get rid of a slightly scratched Dark Hero and the Oni, which he had earlier Lightning Slashed; after Tempest's death, Expat fired Hollow Points in a Submachine Gun to get rid of a second Macabre Spectre among other things, but this left Visionary the Dreamer down to 1 HP (after a previous firing of Submachine Gun and one of Squall from Tempest, who had skipped it on his first turn and wasn't about to die without at least using it once). Resisting the temptation to have AZ destroy The Dreamer just to end the madness, I kept playing, and what did I find? River of Lava. Visionary didn't have Equipment, so she dropped to 1 HP, where Expat already was after the last round of beatdown, and then Grotesque Arachnoid finished us off.
I'm pretty sure Dreamy is the only villain I've played three solitaire games against without even going to Advanced; they've been a 5, a 3, and a 4-hero game, and out of those 12 heroes, 9 have died. (AZ and Tempest were among the casualties both times, along with one instance each of Bunker, O-X, Haka, Expat and Visi.) The only ones who've survived were Fanatic, whose double End of Days singlehandedly won the otherwise hopeless 5-player fight in the Block, and the two heroes besides Bunker who were in the Final Wasteland (one of whom was Chrono-Ranger; I don't recall the other one offhand). Have others found her this tough? She seems to be even nastier than the other minion-based villains who are listed at difficulty 3, and the alternate loss condition (and corresponding impairment of otherwise-desireable global damage effects) is practically overkill. I know she isn't unbeatable per se, but she definitely seems to be a "high" 3 on the scale.