Let's see if this is correct:
Deoxys is faster than Scarf KyuW, so it gets to spike on turn one. Once spikes are down, SR+spikes will instantly KO everything Team 2 has upon switching in (KyuW takes 37,5% due to SR weak, others 25%). In addition, if the opponent switches in Zekrom or Arceus, they will be left with less than 20% after SR, so their own life orb will KO them before they get 3 KOs. So the only relevant case is that KyuW stays in.
Fusion flare, however, OHKOes everything on Team 1, so T1 has to:
- Sac Deoxys-S. This probably happens on the spiking turn.
- Bring Forry in on the KO, taking 12,5% from SR.
- Switch to Mewtwo and sac it.
- Bring Forry back in on the KO to take 12,5% from SR
- Now Forry is at around 23%, so Custap activates and you can SE Gyro Ball for the KO.