So... we got two very powerful special attackers. One is very fast and strong, while the other is even stronger and bulky. Fun. And of course, anything that resists the stupid strong Fleur Cannon from Magearna is going to be at least partially weak to either Psychic or Focus Blast from Tapu Lele, baring Kitsunoh or Doublade, neither of whom can actually do the job. Seeing as Chansey is a piece of shit and not a fun answer, I tried to find something else. Which was hard. Very hard. Almost everything I tried either couldn't take two hits or couldn't kill fast enough in return. But... I found one thing. Well, two things, but one was really dumb. So here is the less dumb one:
Cresselia @ Leftovers
Ability: Levitate
EVs: 252 HP / 240 Def / 16 SpD
Calm Nature
- Psychic
- Ice Beam
- Calm Mind
- Moonlight
With this spread, Cresselia can take absolutely anything that is thrown at it from either of these two without issue. The specific spread gives it just enough special bulk to never be OHKOd by a crit Fleur Cannon, because I just wanted to be sure this always works. It can come in on anything Magearna throws at it, and either heal itself up, or boost with Calm Mind. It may not do a lot in return, but, with boosting, it will eventually be taking nothing and gaining respectable damage output. Tapu Lele is just as simple, as it is simply weaker, while just as choice locked.
252 SpA Tapu Lele Moonblast vs. 252 HP / 16+ SpD Cresselia: 111-132 (25 - 29.7%) -- 0% chance to 4HKO after Leftovers recovery
252+ SpA Choice Specs Magearna Fleur Cannon vs. 252 HP / 16+ SpD Cresselia: 250-295 (56.3 - 66.4%) -- guaranteed 2HKO after Leftovers recovery (except not really, cause SpA drop)
252+ SpA Choice Specs Magearna Fleur Cannon vs. 252 HP / 16+ SpD Cresselia on a critical hit: 375-442 (84.4 - 99.5%) -- guaranteed 2HKO after Leftovers recovery (and again, not really, baring another crit)
252+ SpA Choice Specs Magearna Flash Cannon vs. 252 HP / 16+ SpD Cresselia: 154-183 (34.6 - 41.2%) -- 68.1% chance to 3HKO after Leftovers recovery
Not gonna bother with offensive calcs, since it barely does anything, but the entire point is to stall and boost until it doesn't matter.