In truth only Meloetta and Chansey need to be calc'd and they're both 2HKOed by Boomburst.
Meloetta @ Choice Specs
Trait: Serene Grace
4 HP / 252 Spa / 252 Spe
Modest Nature
- Boomburst
- Psystrike
- Shadow Ball
- Focus Blast
Meloetta is usually considered to be a wall, and its offensive role is usually relegated to late game Shell Smash cleaner. However, Meloetta has one of the strongest Boombursts in the meta (even SpDef Melo does minimum 70% with it to a lot of offensive mons). With Specs and a bit of prediction almost nothing wants to switch into it, and it's unexpected and get a surprise KO when it reveals Specs.
Here's a wall of calcs for everything in the S, A, and B ranks that's a special wall, resists its dual STABs, or would otherwise want to switch into it:
252+ SpA Choice Specs Meloetta Focus Blast vs. 248 HP / 220+ SpD Heatran: 304-358 (78.9 - 92.9%) -- guaranteed 2HKO after Leftovers recovery
252+ SpA Choice Specs Meloetta Focus Blast vs. 248 HP / 8 SpD Mega Tyranitar in Sand: 484-572 (120 - 141.9%) -- guaranteed OHKO
252+ SpA Choice Specs Meloetta Psystrike vs. 4 HP / 252+ Def Eviolite Chansey: 354-417 (55.1 - 64.9%) -- guaranteed 2HKO
252+ SpA Choice Specs Meloetta Boomburst vs. 248 HP / 100 SpD Mega Scizor: 168-198 (48.9 - 57.7%) -- 96.1% chance to 2HKO
252+ SpA Choice Specs Meloetta Boomburst vs. 252 HP / 4 SpD Skarmory: 248-292 (74.2 - 87.4%) -- guaranteed 2HKO after Leftovers recovery
252+ SpA Choice Specs Meloetta Shadow Ball vs. 252 HP / 0 SpD Aegislash-Shield: 202-238 (62.3 - 73.4%) -- guaranteed 2HKO after Leftovers recovery
252+ SpA Choice Specs Meloetta Boomburst vs. 252 HP / 252+ SpD Meloetta: 226-267 (55.9 - 66%) -- guaranteed 2HKO after Leftovers recovery
252+ SpA Choice Specs Meloetta Focus Blast vs. 252 HP / 168 SpD Ferrothorn: 326-384 (92.6 - 109%) -- 56.3% chance to OHKO
252+ SpA Choice Specs Meloetta Shadow Ball vs. 0 HP / 4 SpD Jirachi: 282-334 (82.6 - 97.9%) -- guaranteed 2HKO
252+ SpA Choice Specs Meloetta Focus Blast vs. 172 HP / 0 SpD Magnezone: 466-550 (143.8 - 169.7%) -- guaranteed OHKO
252+ SpA Choice Specs Meloetta Boomburst vs. 0 HP / 0 SpD Mega Gardevoir: 288-339 (103.9 - 122.3%) -- guaranteed OHKO
252+ SpA Choice Specs Meloetta Focus Blast vs. 0 HP / 0 SpD Excadrill: 606-714 (167.8 - 197.7%) -- guaranteed OHKO
252+ SpA Choice Specs Meloetta Boomburst vs. 252 HP / 4 SpD Slowbro: 447-526 (113.4 - 133.5%) -- guaranteed OHKO
252+ SpA Choice Specs Meloetta Boomburst vs. 248 HP / 8 SpD Assault Vest Slowking: 228-268 (58 - 68.1%) -- guaranteed 2HKO
252+ SpA Choice Specs Meloetta Psystrike vs. 252 HP / 0 Def Diancie: 187-222 (61.5 - 73%) -- guaranteed 2HKO after Leftovers recovery
I skipped some things like Venusaur-Mega that are weak to Psychic or something along that, even if they otherwise fit that criteria. I might have otherwise missed some things, but I think you get an idea of this thing's power.
Boomburst spam for days!