Powered Up Mr. Duncan in Rain Hazard Suppression 2.7 (Reverse: 1999)
Team: Flutterpage (R10 / P0), Mercuria (R10 / P1), Lopera (R10 / P0), Fatutu (R10 / P0), Mr. Duncan (R10 + P5)
Hot take, but I think Duncan is actually pretty good in this mode. Duncan's main problem has always been "he can't force enemies to target him". But When the Alarm Sounds has a lot of Everyday Patrol stages where enemies get to spam their Mass Ults every other turn. While other characters are probably sweating at the idea of tanking back-to-back Mass Ults, Duncan is just rubbing his hands together, because he knows he's getting Counters that turn. If you're really lucky, he'll just snipe and kill the enemy with his Counter before the Ult actually lands.
I think it also helps to treat Duncan as less of a "Bullet" character (because "Bullet Team" just isn't really a thing in proper endgame content) and more of a "Beast Afflatus Follow Up" unit. As a team, Flutterpage, Fatutu, Duncan and Lopera is surprisingly underrated. In teams where you would need a Beast unit, Flutterpage would have Afflatus disadvantage. But Fatutu can help with sustain, while Duncan really benefits from Flutterpage (both her FUA Crit boosts and FP's FUA since Duncan gains Ceremonious stats when allies activate Bullet effects, which happens when running Lopera and Duncan with FP). Of course, Anjo Nala is a far better Beast FUA character but it still works out because we have multi-team content in R1999, so it's not that unrealistic to have Anjo and FP on different teams.
As for this specific stage, Duncan is really lucky. Even though he's not getting an Incantation/Counter boost from Lopera, he really doesn't need it. This is because Stress Ults in this mode actually count as a Follow Up Attack (you can tell because Flutterpage's Eureka gets drained when the Stress Ult gets used). Between Flutterpage and Lopera's support and this stage's specific Slot 5 buffs, you'll see absolutely ridiculous Mr. Duncan Ult damage.
Overall, I think this was a good patch for the old man.