Well Mega Gardevoir hates Mega Mawile, Mega Gengar, Aegislash, Talonflame, and Mega Lucario if it gets in safely. Tyranitar can only beat Talonflame and maybe Aegislash, so they're all things that you're going to have to look out for. Scarf Landorus-T can do work against all of those, the aforementioned Heatran can be decent, Garchomp is also great but shouldn't be used on the same team as Lando. Rotom-W is pretty good, Excadrill pairs with Tyranitar nicely. You'll also want something to check Azumarill since that can Belly Drum and sweep against most of these listed Pokemon. Serperior comes to mind as a good stop to that...
the good
252 Atk Landorus-T Earthquake vs. 252 HP / 0 Def Mega Mawile: 576-680 (366.8 - 433.1%) -- guaranteed OHKO
252 Atk Landorus-T Earthquake vs. 0 HP / 4 Def Mega Lucario: 392-464 (139.5 - 165.1%) -- guaranteed OHKO
252 Atk Landorus-T Earthquake vs. 0 HP / 4 Def Mega Gengar: 422-500 (161.6 - 191.5%) -- guaranteed OHKO
the bad
252 Atk Landorus-T Earthquake vs. 248 HP / 16+ Def Mega Scizor: 118-141 (34.4 - 41.1%) -- guaranteed 3HKO
although ttar can deal with it
252+ Atk Technician Mega Scizor Bullet Punch vs. 248 HP / 80+ Def Tyranitar: 186-222 (46.1 - 55%) -- 61.3% chance to 2HKO
0 SpA Tyranitar Fire Blast vs. 248 HP / 200 SpD Mega Scizor: 252-300 (73.4 - 87.4%) -- guaranteed 2HKO
and guaranteed ohk with minimal hp/def investment
Well, my Aegislash with sacred sword and iron head can demolish both pokemon. As you say, steel types in general are a problem cause metagross and scizor can demolish them too.
Edit: ninjad and i also really think heatran is a great answer to those pokemon except for the rare earthquake metagross but you shouldnt worry too much about that and you can check it anyway. I however also kinda like mandibuzz for the above pokemon with maybe swagger and foul play, although mandibuzz isnt very common.
yeah heatran is a great answer to aegislash
252 SpA Heatran Magma Storm vs. 252 HP / 252+ SpD Aegislash-Shield: 176-210 (54.3 - 64.8%) -- guaranteed 2HKO after Leftovers recovery
Mega-Mawile can tear through both of those in an instant, so I'd watch out for that. If I were you i'd bring in Heatran, as it's a great partner with M-gardevoir. Garchomp fits nicely and is a great overall threat, that could be a decent 3core for you.
Garchomp > ttar , I agree with you here bc
0- SpA Garchomp Fire Blast vs. 248 HP / 200 SpD Mega Scizor: 196-232 (57.1 - 67.6%) -- guaranteed 2HKO
0 Atk Garchomp Earthquake vs. 252 HP / 4 Def Aegislash-Shield: 188-224 (58 - 69.1%) -- guaranteed 2HKO after Leftovers recovery
but I prefer to run ttar since he gives me a check to tflame and scarf lando t does the same job as garchomp
So all in all I my team would look like this
(All out attacker) M
(psyshock, hyper voice, focus blast(?) , ??)
SpDefensive Support
(leftovers, shuka berry ?)
Scarf
(eq, knock off, u-turn, ??)
??
(magma storm/eruption/fire blast whatever that suits my team)
Since steel types are the strongest threat to MGarde Ttar I opt to run 2 checks. But this way water types like azumaril, rotom w, suicine, (m) slowbro and several others become another serious threat. Anyway, Hoopa U might be a good addition to deal with those.
Azumaril, slowbro, rotom w
252 SpA Choice Specs Hoopa Unbound Thunderbolt vs. 240 HP / 16 SpD Assault Vest Azumarill: 282-334 (70.3 - 83.2%) -- guaranteed 2HKO
for those rare occasions, otherwise always a ohk/revenge kill from lando
252 SpA Choice Specs Hoopa Unbound Dark Pulse vs. +1 252 HP / 176 SpD Mega Slowbro: 314-372 (79.6 - 94.4%) -- guaranteed 2HKO
incase he doesnt cm when I switch in hoopa u... 136+ SpA Mega Slowbro Scald vs. 0 HP / 0 SpD Hoopa Unbound: 106-126 (35.2 - 41.8%) -- guaranteed 3HKO
...kill/forcing switch in next round
252 SpA Choice Specs Hoopa Unbound Dark Pulse vs. 252 HP / 40 SpD Rotom-W: 219-258 (72 - 84.8%) -- guaranteed 2HKO
116+ SpA Rotom-W Hydro Pump vs. 0 HP / 0 SpD Hoopa Unbound: 121-144 (40.1 - 47.8%) -- guaranteed 3HKO
252 SpA Choice Specs Hoopa Unbound Thunderbolt vs. +1 252 HP / 4 SpD Suicune: 214-252 (52.9 - 62.3%) -- 99.6% chance to 2HKO after Leftovers recovery
Also it works against crocune, but if luck plays a big roll. If it gets a few lucky sleep talks my game is most likely done. Though running a taunt breaker heatran might solve this problem and help against other stall mons/comps.
This leaves me with one space left. Wish support for Hoopa U and MGard ? Or ditch ttar completely for something else to cover other weaknesses, then let lando support with SR ? Or am I just crazy to think this comp might work ? I am open for every kind of critic, guys.