Bubble strat, confirmed

The following pairs are confirmed to work. Confirmed as in I've been using them myself.

• Gastly / Astonish CP 40:ish (12 or lower HP) -- Gastly / Lick CP 24
• Gastly / Astonish CP 40:ish (12 or lower HP) -- Haunter / Shadow Claw CP 20
• Raichu / Volt Switch CP 25:ish (12 or lower HP) -- Diglett / Mud Slap CP 11 (lvl 1.5)

Since the Haunter runs for 1000 prestige it's better to evolve a small gastly, but sometimes you just don't get them.

The Raichu defender more or less requires you to have caught an insane number of pikachu during the events with pikachu and hats. You evolve a CP 10 pikachu and pray for Volt Switch.

Oh, and don't forget that the diglett has to be powered up once. I did, and it kind of got expensive in terms of revives and potions.

There should be more working pairs out there, but these are the ones I've found.