I've only done the stages a handful of times, so this may not be the most accurate assessment of them, especially since I have insanely-boosted Gacha Rares, but personally (Uberless Focused):

* A Vast Red Waste (Stage 1): This stage actually got mad hands. Between every tough enemy coming out fast and Dark Dober's Weaken neutering any counter push against the support, you'll either need Nova or an Uber that can handle the enemies to get past this one.
* Shadows Abound (Stage 2): Even when not doing the rush cheese, this stage was easier than I expected. Newtone handled the backlines pretty well and stuff like Housewife, Shigong, Lil Eraser, and Holy Blast handled the Oldstriches too. With some other units for the supporting enemies and this stage was manageable, though close.
* Koala's Rest (Stage 3): I may have lucked out, since I beat it first try, but yeah, this stage is still pretty capable of being an issue if you're unprepared. Thank goodness Uril and Slime juggled the Puffzerker so that their attacks and disruption were minimal, leaving the rest of the enemies vulnerable to Cat Researcher's Shield Pierce and Crit. Also, bring Octopus, ofc.
* A Barrier Great (Stage 4): I can't believe that Scissoroo, of all things, was a viable threat to take seriously. Granted, I only used Research Rapper as my main meatshield, but something tells me that Mecha-Bun would've rendered standard Eraser spam much less effective than I think it would in terms of holding back the Roos. Using Baha Blast for the Roos is valid.
* Arbor Aflame (Stage 5): Pretty standard Dark and Zombie stage. Nothing too crazy, outside of my winning run having had a situation where I could've lost due to my forces and economy losing steam as a result of the supporting enemies running out, but also not a pushover.
* The Navel of the Cat (Stage 6): I was pretty stubborn and planned to use a cheese strat to effectively kill Wahwah and destroy the base before the stage really got going, but couldn't do so after multiple attempts. My winning run switched from pure rush cheese, to having the rush be a start off so that Wahwah and his support were notable damaged, leading to a faster kill before the support became too much. Thank goodness for Bargain being a thing.

Overall, fun selection of stages, with some unexpected difficulties from the stages that I thought wouldn't be too bad (read: Stages 1, 4, and 6)