Question

Is there a way to defeat every Legendary Pokemon with a single Pokemon?

Comments

It's pretty easy. All you need is a mega Gengar with some coverage moves, and have all the legendaries only know normal attacks. It's even easier if the legendary Pokemon are underleveled.

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An example for this would be my Incineroar taking down Necrozma. Since Incineroar is immune to Necrozma's Prismatic Laser (the most powerful, non "Z-move" Psychic-type move in the game) and I used Incineroar's Darkest Lariat / Leech Life move(s) to defeat it.

Answer 1

For the one Pokemon, it would probably be best to use a Rayquaza with this set.
Rayquaza @ leppa berry
EVs: 4 HP / 252 Atk / 252 Spe
Adamant nature
-swords dance
-dragon ascent
-outrage
-earthquake
The three moves achive neutral coverage against all legendary Pokemon, and dragon ascent is necessary to mega evolve the Rayquaza. Max physical attack, leppa berry, and swords dance decrease Rayquaza's chance of running out of PP. The other legendaries should all be level 70 (or as low as possible), holding toxic orbs, and knowing confide as their only move.