Stakataka
Stakataka is a weird one. Firstly, I'd recommend getting rid of Trick Room unless you're going to make the entire team a Trick Room team. If Staka dies, then Trick Room will probably hinder the rest of your team, especially Jirachi, Corviknight, and Excadrill, which are all supposed to be speedy.
Secondly, Aguav Berry isn't the best item for Stakataka. Most sets run Assault Vest, which ups Stakataka's lower Special Defense. This would work well with the removal of Trick Room.
Stakataka @ Assault Vest
Ability: Beast Boost
EVs: 180 HP / 252 Atk / 72 Def / 4 SpD
Adamant Nature
- Body Press
- Heat Crash / Zen Headbutt
- Gyro Ball
- Earthquake / Stone Edge
Corviknight
So......I see you went with unconventional. A lot of Corviknight sets run Bulk Up, to make it (obviously) bulkier and hit harder. It works even better if you run Body Press. The 4 moves Corviknight uses the most in Monotype (see here) are Roost, Bulk Up, Brave Bird, and Body Press. this set provides decent coverage and with enough Bulk Ups, can turn Corviknight into a monster. Corviknight also has a nice ability Mirror Armor to stop Intimidate pivots like Landorus-T.
Corviknight @ Leftovers
Ability: Mirror Armor
EVs: 252 Atk / 252 Def / 4 SpD
Impish Nature
- Roost
- Bulk Up
- Brave Bird
- Body Press
Excadrill
Excadrill looks perfectly fine and is pretty standard, the only thing I would suggest is replacing Rocky Helmet with Leftovers. I assume it's your lead?
Heatran
Heatran on this team is a great candidate for Choice Scarf. You don't have a Scarfer already, and Heatran is willing and ready with nice coverage and decent defenses. Choice Scarf Flamethrower can destroy many frequently used Pokemon in Monotype. It gets a guaranteed OHKO on Ferrothorn, and Aegislash-Blade, and can 2HKO Autotomize Celesteela and specially defensive Excadrill. Because of it's useful ability in Flash Fire, it can switch into a Fire attack for another member and up the power of it's own Fire moves.
Heatran @ Choice Scarf
Ability: Flash Fire
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Flamethrower
- Earth Power
- Flamethrower
- Dark Pulse / Dragon Pulse
Melmetal
Melmetal's standard random set uses Substitute, so maybe you should consider that instead of Toxic to prevent burns. Personally, I would go with a full-on Choice Band set, to make the most use of it's tremendous Attack stat. Thunder/Ice Punch are boosted by Iron Fist, and together make the BoltBeam combo. Most others run this set in Monotype as well.
Melmetal @ Choice Band
Ability: Iron Fist
EVs: 240 HP / 252 Atk / 16 Spe
Adamant Nature
- Double Iron Bash
- Thunder Punch
- Ice Punch
- Earthquake
Jirachi
I see why you made Jirachi a special attacker on this set, because your team doesn't have many. The standard Body Slam + Iron Head is nice, but a Calm Mind Jirachi is unexpected. I would also consider running Energy Ball just for plain coverage. Keep Moonblast because it will have a 60% chance to lower the foe's special attack, and because you don't have any other Fairy-type coverage. I would also run Thunderbolt (for the 20% chance to paralyze) or Shadow Ball (40% chance to lower the opponent's special defense).
Jirachi @ Leftovers
Ability: Serene Grace
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
Shiny: Yes
IVs: 0 Atk
- Calm Mind
- Energy Ball
- Moonblast
- Thunderbolt / Shadow Ball
Of course, take my RMT with a grain of salt and use whatever you want to use. If you feel like running Agility Corviknight, Wynaut? Go wild. You can find my version of your team in this Pokepaste.
Edit: Turns out Moonblast and Ancient Power were incompatible, so I replaced it with Energy Ball. The Jirachi has to be shiny to use Energy Ball, by the way.
