For the first time in ages, I've completed an escalation battle. For once, it's been a quick and pleasant experience, with no failures on regular stages and no coins wasted. Just in case somebody needs advice, these are the teams/items I used for boss stages:
Megas: Absol (15, 8/8, SL4), Rayquaza (14, 20/20), S-Tyranitar (6, 4/4), Tyranitar (10, 15/15, SL2)
Supports: Yveltal (15, BS+ SL2), Hoopa-U (10, SL5), Dusknoir (12, SL5), Darkrai (10, SL3), Genesect (15, SL4), Zoroark (13, SL2)
25/50: very easy stages, any team can beat these.
75: Absol, Yveltal, Hoopa-U, Dusknoir - Itemless - 2 moves left
100: Rayquaza, Absol, Darkrai, Dusknoir - Itemless - 2 moves left
125: S-Tyranitar, Genesect, Hoopa-U, Dusknoir - Itemless - 1 move left - This stage actually gave me troubles, I failed it 3 times thanks to RNG (no combos, no LDE activations on mo3, absurdly low damage rolls with RT - and with absurdly low I mean 367 damage on a mo5...). Nonetheless, very doable with just average luck. Maybe bring Diancie instead of Tyranitar for faster mega evolution.
150: Tyranitar, Dusknoir, Darkrai, Hoopa-U - MS, DD, M+5 - 2 moves left - This stage scared me a bit because of the nasty disruption and the progressively lower disruption counter. Luckily, Darkrai did its job after the DD expired, so Meloetta disrupted me only once and I could comfortably finish her with Dusknoir's LDE. If you feel that your team lacks power (especially if you didn't invest in Hoopa-U or Dusknoir) don't be afraid to add an AP+.
175: Tyranitar, Zoroark, Darkrai - C-1, MS, M+5 - 9 moves left - C-1 makes this potentially troublesome stage a walk in the park. If you feel brave, you can probably skip the MS. More skilled players can succeed with the "full items bar C-1" strategy, but I wouldn't recommend it. If you still feel like trying it, remember that it's vital to put Meloetta to sleep after the DD expires, since the stage has really awful disruptions.
200: Tyranitar, Dusknoir, Darkrai, Hoopa-U - All items - 2 moves left - During the first 7-8 turns everything went wrong. The starting board had only 3 unmatchable Tyranitar icons, so cleaning the board took me 4 turns, during which I dealt next to no damage. Moreover, Hoopa-U refused to collaborate (as usual). Luckily, I was able to recover during the second half, thanks to Darkrai (never saw a disruption) and Hoopa-U (2x5000+ damage). Dusknoir delivered the final blow with a mo5.
MVP: Darkrai and Dusknoir ex aequo. Darkrai was super useful to block disruptions during boss stages, while Dusknoir made high-HP stages much more manageable. Investing in Dusknoir (SL4 works fine) is strongly recommended, you won't regret it.