I wouldn't power up an arcanine for the purpose of defending. A vaporeon can beat them while taking very little damage.
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yes, since it can be beaten easily, that means you can train the gym effectively
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I would not. I have 91% fire fang/flame and I did not power up. Stardust is very rare to spend in this type of pokemon ( not perfect moveset and under 90% iv )
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Fire type has it to hard in meta. Keep as is to attack exeggutor.
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I have only 90k Dust. I need dust to powerup more important pokemon. Until I have dust to power up my Arcanine I will ( prob ) have a better Arcanine.
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Have you used it ? I don't know why everybody is obsessed with Fire Blast. Fire Fang/Flame is a killer combination, my Arcanine 91% have that moveset and honestly it kicks ass.
I also have a 84% Arcanine with Bite/Fire Blast and don't find it as useful as my other one.
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The problem is not about Arcanine, It is about the rarity of StarDust.