[RATE MY TEAM] try this on for size

[Marcusube]

If this is a VGC team then you're gonna have to consider revamping these move sets.

For starters, Tentacruel's moveset is too singles friendly and too much stall. For it to support Altaria, you need to have it learn some speed controlling moves.

Altaria, being the core of your team, needs to have moves which allow it to stick around longer. Moves like Protect and Roost really helps it to survive in the heat of battle.

Saying my statement about Greninja is false puts you in this kind of position

-1 252 Atk Expert Belt Protean Greninja Ice Punch vs. 4 HP / 0 Def Mega Salamence: 134-163 (78.3 - 95.3%) -- guaranteed 2HKO
252 Atk Protean Greninja U-turn vs. 4 HP / 0 Def Bisharp: 49-58 (34.7 - 41.1%) -- guaranteed 3HKO

You have a point but to counter the highest threats in the VGC meta game, you can't settle for anything less than the strongest set.

Intimidate teams generally run moves that hit Empoleon super effectively, for example

252+ Atk Landorus-T Earthquake vs. 248 HP / 0 Def Empoleon: 168-200 (88.4 - 105.2%) -- 31.3% chance to OHKO
252+ Atk Mega Salamence Earthquake vs. 248 HP / 0 Def Empoleon: 112-134 (58.9 - 70.5%) -- guaranteed 2HKO

The only thing on your team that can stop an Intimidate team is the Greninja but since it is physical, it falls victim to getting taken out early in the game.

252+ Atk Life Orb Talonflame Brave Bird vs. 252 HP / 0 Def Amoonguss: 283-338 (128 - 152.9%) -- guaranteed OHKO
252+ SpA Mega Charizard Y Heat Wave vs. 252 HP / 252+ SpD Amoonguss in Sun: 192-228 (86.8 - 103.1%) -- 18.8% chance to OHKO

Not saying you shouldn't run the Amoonguss but just reminding you that your team may have trouble checking these Pokemon.

Finally, using Rotom as an all out special attacker may hinder than help because it is slow in comparison to what can kill it with ease and it isn't the most powerful option out there.

Tentacruel @ Black Sludge
Ability: Liquid Ooze
Level: 50
EVs: 220 HP / 196 SpD
Bold Nature
- Muddy Water
- Icy Wind
- Protect
- Confuse Ray

Altaria-Mega @ Altarianite
Ability: Pixilate
Level: 50
EVs: 236 HP / 100 Def / 76 SpA / 92 SpD / 4 Spe
Modest Nature
IVs: 0 Atk
- Protect
- Roost
- Hyper Voice
- Fire Blast

Greninja @ Life Orb
Ability: Protean
Level: 50
EVs: 20 Atk / 236 SpA / 252 Spe
Hasty Nature
- Gunk Shot
- Protect
- Ice Beam
- Dark Pulse

Rotom-Heat @ Sitrus Berry
Ability: Levitate
Level: 50
EVs: 196 HP / 156 Def / 64 SpA / 92 SpD
Bold Nature
- Will-O-Wisp
- Thunderbolt
- Overheat
- Protect

This is what I would change on your team.