How did i get my yveltal to 1hp

i defeated a ghost type team go rocket grunt and turns out i somehow got my yvetal to 1hp after the grunts froslass nearly cooked me (it was their last pokemon)

i aint braggin or sth js curious if getting a pokemon to 1hp is uncommon cuz every time i thought a pokemon was at low hp it was at 2 or 3 hp