Ok, so with this ability, Overheat and Close Combat are pretty much going to be standard on it. The other viable moves that it could run (given the nature of Arcanine) are: Morning Sun, Extremespeed, Crunch, Wild Charge, and for this case, Rest and Sleep Talk. For Special Moves: Dragon Pulse, Hidden Power, and then Hyper Beam and Snarl - the last two are basically unused.
Given this information, let's thinking about the non-Pokémon-specific to this type of Arcanine. Anything immune to Fire that can soak up physical hits could work with Toxic, as long as Arcanine isn't running Rest (even then, Taunt exists). Also note that it can't boost its offensive and defense at the same time.
So we have:
1. Mainly Physical Attacker.
2. Can only boost Special Attack or Defenses (but not both) in one turn.
3. Doesn't boost Attack or Speed.
I mean first of all, Unaware 'mons would stop this. Even if Arcanine is running recovery, Calm Mind Clefable would destroy it. You'd have to run Toxic just to beat Clefable or Quagsire.
Secondly, it's not going to be immediately threatening. Like Serperior, it's non-threatening with 0 boosts. Given the many weaknesses of Fire/Fighting (and the fact that it can't have an item), even if you boost once you're not going to be unstoppable after one SpAtk boost or unkillable with one Def/SpDef boost. The safest way you can come in is after a sack on a 'mon who is slower than you who would get destroyed by Overheat/CC, so Amoonguss or Bisharp, let's say. Suppose you kill it, and then get either a defense boost or +2 SpAtk. Then they go into something that's faster and can hit hard. Scarf Lando (doesn't even have to be scarfed if we lower speed), Specs Keldeo, Talonflame, etc. If you got the defensive boosts, then you won't do damage with your next attack. If you got the SpAtk boost, then you won't be living a hit. The only "problem" I see is that it forces the opponent to sack a Pokémon when you bring it in, but that's the case with a lot of Pokémon.
Also it's not going to boost Attack or Speed. Most of Arcanine's attacks are physical and it's also not very fast. It's not going to be a fast devastating sweeper, but it's more of a bulky sweeper like Mega Slowbro. It's offensive coverage would be low because it's only going to be using Overheat, Dragon Pulse, and probably HP Grass (wow, that's almost the same as Serperior!) It only beats Heatran because of Close Combat, otherwise it would get walled as well. The only way attack is going to get boosted is if Arcanine gets intimidated.
Knowing this, here is what I suggest:
HP: 90 (+0)
Atk: 70 (-40)
Def: 111 (+31)
SpAtk: 165 (+65)
SpDef: 111 (+31)
Speed: 98 (+3)
BST: 645 (+90)
(If BST Growth requirement can be lowered, I would gladly lower)
So I'm really going to take the Arcane part seriously - the high SpAtk really means that Arcanine is now a wizard dog. This spread means that you'll likely be running Overheat, Dragon Pulse, Hidden Power, and either Morning Sun for recovery or Close Combat if you want defenses (most likely Close Combat).
Even if you're at +1 Defenses with no investment (and assuming Rocks are up and you switch into them as normal Arcanine), Timid Scarf Lando has <50% chance to OHKO, Jolly Sharp Beak Talon has a small chance to OHKO, but the situation is different if Talon is Adamant or running Band. Keldeo Hydro still OHKOs even without Rocks, so does Medicham Fake Out + Zen Headbutt, and also Mega Pinsir Return (provided you've Mega Evolved already). Keep in mind that this is all provided that you're at +1 for defenses, which means that you've already used Close Combat on a sack or something slower and weak to it, and that you probably don't have +2 SpAtk.
The important things are:
- Mega Arcanine has poor attack, so running a Physical set with Overheat might be possible, but deals less overall damage (since you have bad attack and your only move to benefit from boosts would be Overheat)
- It's fairly slow and you cannot boost speed.
- Usually you can only get one boost (+1 Defenses or +2 SpAtk) before the opponent switches into something that kills you. If you got +2 SpAtk then you're not living the hit. If you got +1 Defenses then you still might not live the hit, and your damage is lowered.
- You can't fully invest into Attack, Special Attack, Speed, or Defenses.
I think the best option for this Arcanine is to run some max SpAtk and Max HP to let you take on hits better, but it's going to be a lot slower than usual.
Yes someone can make a bunch of counter-arguments because I used one specific set (Timid 252 SpA, 4 SpD, 252 Spe) for calculations, but my point is that you can't boost offense and defense simultaneously. There aren't many things that can wall this set, but there are a lot of things that can revenge kill it.
Also compared to some of the other sweepers, this is nothing. Zard X, Dragonite, and Mega Pinsir can all destroy teams once set up, because they're going to outspeed your entire team. Even a Baton Passed Manaphy or Mega Heracross can do the same. Mega Arcanine can't because it can't boost speed (I mean, I guess Baton Pass + Mega Arcanine is deadly, but just ban BP).