In my first Status Moves only playthrough (I wonder when someone will say, "We get it, you're a pacifist.") I felt pretty clever for waiting until Cloyster for Spikes before taking on the Grass Gym and using Roar to do what Leech Seed and Poison Powder couldn't and finishing them off with Butterfree's Supersonic... Then someone pointed out I could've caught Gastly for Curse. I even remembered thinking about Gastly and deciding not to use it.
In Soul Silver, the Psychic Elite four KOed my Meganium, Ninetales, and Forretress and he was down to just Slowbro. Lugia couldn't do anything, but I'd brought the Level ~30 Togetic I'd given up on and a Level 8 Gastly I'd used for the first gym just to get them into the hall of fame because they'd helped me in the early game. Gastly could use Curse, but he'd die if he took a hit. I switched to Togetic instead and, to my amazement, Slowbro used a stat boosting move. Another surprise was Togetic outspeed Slowbro (In hindsight I should have seen that coming because Slowbro has slow in its name.) which let him use Encore. That let me switch to Gastly and use Curse without getting KOed. I then let him get KOed so he wouldn't take Exp away from Lugia.
This wasn't actually big brain, but I'm bringing it anyway because it felt like "They can't take X if you don't have X" meme big brain. In Shield the Dark Gym's Scrafty used Sand-Attack against my Toapex. Toxapex's Moveset was Toxic Spikes/Baleful Bunker/Recover/Hail. None of those moves directly affected them. (or they at least affects them is an even less direct way than usual.)
I'm not sure if I saved the best for last or the one that shouldn't count for last. There was no way this would normally happen, even in a challenge run.
A massive Small Brain thing I did was grinded at the Battle Frontier in Soul Silver just because it was the only way to get the TM for Toxic. (I wouldn't have done that if there was a Toxic TM anywhere else in the game, but there wasn't, so I grinded for 3 of them.) Somehow beating 4 of the 5 Frontier Brains would be the highlight of the playthrough but it was only "half" over so I planned a Massive Big Brain Combo combo to end with a bang. I KOed Red's Charizard in One Turn with Status Moves Only. Forretress set up Stealth Rock, (8/16) 3 Layers of Spikes, (4/16) 1 layer of Toxic Spikes, (2/16) Lunatone's Gravity (which took grinding for even more BP) let every entry hazard hurt, Meganium lured in Charizard by being a Grass type and endured Blast Burn with an Occa Berry and retaliated with Leech Seed, (2/16) and Tyranitar's Sands Stream (1/16) chipped off the last bit of HP left. (because of rounding, I needed 17/16ths instead of 16/16)