This can apply to online, with your friends or in game battles (either trainer or just wild pokemon) but what complete disaster (for the opponents) worked out in your favour spectacularly?
Mine happened the other day when I was facing a Greninja and Gardevoir and I had out a Golurk and Qwilfish with Poison Point and holding Rocky Helmet. I figured the Greninja would attack Golurk with Ice Beam or Scald (never even though of Dark Pulse, but that could also have been a possibility) and even if it wasn't Protean (which I have seen a surprising amount lately) it would still be Dark Type at least so I took a gamble and used Drain Punch on it, and had Qwilfish layout Toxic Spikes.
The Opposing Greninja used U-Turn on Qwilfish (which I hate how every single moveset online always has this move on Greninja, speaking from experience, it literally does almost no damage to the opponent (me!) and since Greninja is fast, it's literally exactly the same order as switching out normally. anyway, I digress) and it got Rocky Helmet damage AND got poisoned from Qwilfish, switched out and Lucario came in only to be KOed by Golurk's Drain Punch. I don't recall how the rest of the battle went, (the first Gardevoir turned out to be a Zoroark which I didn't know till the second Gardevoir came out) but I easily trounced them with just my Golurk and Qwilfish.
Need a specific Berry? I have literally every kind. Need a Friend Safari HA pokemon? I have them all. Need a gen 1 - 3 (and a few from gen 4) Move Tutor Egg Move that can no longer be obtained? I might be able to help. Drop me a DM if you need anything and maybe I can help!
My first battle in a competition, January 2016: I call this "never give up!" :D YNUW-WWWW-WW3F-FMAQ