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(Additional stuff, I don't think M-Aggron is a good late game sweeper imo. It needs those SpA sweepers gone, but it definitely won't sweep. Maybe bulky attacker is more precise)
Aranines defense is mediocre, a 2 EQ from an Invested Aggron will take him out, even with Morning Sun. which Aranine losses a turn to set up( giving you time to get toxic off). (+Filter is going to stop him from taking as much dam from Stab+super flare blitz). Chandelure does generally does run speed, i just warned him to invest otherwise he would fall to a scald from vaporeon. (Though i guess ability Flash Fire would work on a late game Arcanine as well). Finaly, Mega Aggron will sweep only if he isint switching into a poke who has not set up. If he can get in without having to deal with any setters, his bulk alone lets him just outlast everyone. (I know it works, ive used M-Aggron to outlast teams, including Sweeper Arcanine, though i always only get through just barely).
Edit: Note also i dont bother investing in SpD for Mega Aggron as smogon adivses, because he will almost always lose to a SpD even with the Evs. Putting them in Attack makes him hit harder and remove threats quicker. (Only if Used late)
Thats a good point, i forgot about will-o-wisp XD. If get wil-o-wisp off on any physical its game over anyway. How long will it take for flare blitz STAB+Super take to get through Invested HP, Filter, and Def+16 On Mega Aggron?
But you lose a turn to set up Moning, if your FlareBlitzing you lose health, counteracting the health regen. Assuming it would take 3 hits, You've already lost by 1 turn to set up Morning, unless he used that turn to set up toxic, in which case on the third turn you take increased dam+the past turn dam. (However, Willo will wipe out MA No matter what).
When you said "set up morning" do you mean setting up the sun or just healing? Healing is almost never a bad option, only exception is when you're PP stalling
Just healing, i dont think he has a way to set up environment. If you take the turn to heal MA has time to get a get another EQ, or use a turn to get of toxic. Third turn, Assuming no Burn(Which should have happened already anway, neutering MA and making him useless) Toxic will take away enough health this turn to easily counteract leftovers. Both will die if either gets of their next move. If Arc Heals its game over for him. Toxic 4th will undo MOR. If they go head to head, Arc Outspeeds and wins, FlareBlitz recoil hits Arc, + Left recovers (I think left gets priority, correct me if im wrong) + 3rd turn Pois Ends with Arc either dead, or with almost no health. Whos ever last in your team should be able to outspeed/priority and get the kill easily. (Also, have you been calc Flare Blitz recoil?)
No I don't do Flare Blitz recoil, but should be small enough for defensive 'nine. Defensive 'nine could take any EQ from M-Aggron all day long, shouldn't be much of a problem even if he uses morning sun. Assuming you switch in to EQ, you can burn it on turn two and heal up the damage on turn three. You'll have ~75% HP after healing
That's why, assuming you know your opponent is using Arc, you first turn toxic. I agree Burn will always neuter MA and give you the win, but if your opponent runs toxic instead, MA will win. Assuming switchin, (As you said last post, i wass assuming same turn switch)1st: MA Toxic, Arc:X 2nd: MA: EQ, Arc :(Assuming No Willo) Flare. Turn Calc = MA ~2/3 Arc ~2/3-toxic 3rd: MA: EQ Arc: Heal/Flare. Turn Cacl = Sen 1 = MA: ~2/3, Arc ~5/8-toxic: Sen 2 MA: 1/3 Arc : 1/4. Turn 4 = MA: EQ Arc: Flare/Extreme(Idk Why)/ Morning. Turn Calc = Sen 1 = MA:0 Arc: 0 (Poison) Sen 2 = MA 1/4 Arc: 0 Sen 3 = MA: 1/3 Arc: 0 (Poison Takes him).
defensive 'nine shouldn't be flare blitzing so often, really. It just wastes its HP. Also defensive 'nine runs leftovers to improve longevity.
tl;dr M-Aggron should run toxic if it wants a chance against defensive 'nine, or hope that you para hax it with T-Wave, rendering it unable to heal and spam EQ like a boss
In all, it doesn't really matter, even if MA is able to take out Arc, his HP is going to be too low to be anything else than cannon fodder. however, left is really used to negate the affects of flare, and gain some hp when hes using status/extreme. toxic is abit better than twave because even if MA losses, it will stop Arc from being effective against the next opponent.
'nine shouldn't be staying in against special attackers anyway. When M-Aggron goes down, 'nine will probably keep spreading burns or just revenge kill very weakened mons with Espeed