Ah yes, Chi-Yu, the embodiment of game freaks tendencies to overtune Pokemon in the recent gens, Chi-yus overheat is one of the most powerful moves in the game when paired with choice specs, Tera fire and sun support thanks to beads of ruin, passively lowering opposing special defense.
Da Funni Feesh
Chi-Yu @ Choice specs
Tera type: fire
Modest nature / timid nature
Evs: 4 Def / 252 Spa / 252 Spe
-Flamethrower
-Overheat
-Dark pulse
-Psychic / Fire Blast
Ah Yes, the classic Spec Chi-Yu, Utterly Annihilates any fat team structure and is even more so under the sun. This set is famously known to be able to nuke blissey from full with Tera fire sun boosted overheat, and was banned from OU almost immediately.
Choice specs Chi-Yu is a reliable source of immediate power for balance and Bulky offense teams, and an utter nuke on sun when paired with a sun setter thanks to its great 135 special attack and a unhinged ability that passively decreases opposing special defense, allowing for more damage.
Flamethrower is the main Stab move, Overheat is the renowned nuke, dark pulse is secondary stab and psychic can hit the fighting types that it may have a hard time with. Fire blast could also be ran for the right situations. A timid nature could also be used if the speed is needed, but modest can hit some benchmarks that timid could not.
Now grab this set and nuke em.