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So, turns out Burn Out actually makes you Typeless. Meaning you literally have no weaknesses.
Also turns out, Arcanine can be one FUCK of a wall if he has no weaknesses.
If you consider Intimidate and give him Assault Vest, his defenses become essentially 90/130/130.
90/130/130 (50% increases to final stats act as increases to base stats by a little more than 50%)
To put this in perspective, that's physically and specially defensive as Uxie or Suicune but with no weaknesses. No fucking weaknesses at ALL. He also gets much better Attacking stats on both sides.
Don't like Assault Vest? That's ok, you can still make him Specially Defensive with his EV's and Nature, and you can carry Will-O-Wisp and Morning Sun to make him the best fucking Support Tank ever, thanks to - you got it - NO WEAKNESSES.
Wish he was Fire Type again for whatever godawful reason you have? Just switch him out and bring him back in later - presto, he's Fire Type again and can use Burn Out once more.
The only issue at this point is his Speed, but as long as you aren't trying to use Burn Out on Excadrill or a Water Type (why the fuck would you), this shouldn't be a problem.