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Flareon is a viable attacker into Blissey, with an 246 base ATK while resisting Blissey's Dazzling Gleam.
With the addition of Overheat, Fire Blast loses to it in every known way. (yellow flash, DPS, energy cost, power, STAB, cooldown) Overheat does eventually massive damage to water Pokémon, which resists it, and is able to kill the big grasses with just 1 hit.
=Attacking=
Heat Wave is a Flamethrower that does more damage with only 1 bar.
Ember, though not elite, is the best quick move for Flareon for dodging.
Fire Spin outputs more damage with only 0.1 more seconds cooldown.
=Defending=
Flamethrower, being the only move with 2 bars that Flareon is able to learn, is likely to hit at least once against water.
Overheat takes too long time to charge for defense. Especially Hydro Pump is now buffed.
Heat Wave and Fire Blast are weaker versions of Overheat at gym defending.
Next post is still Flareon!