If i understand this correctly, you and your cousin are playing a hackmons (unrestricted) national dex format of gen 6?
Because there are many moves on some pokemon that they do not get (like follow me whimsicott and many others). Also, you are using white herb on xerneus while i think you wanted to use power herb (white herb removes the first stat nerfs that you get while power herb makes the first 2 turn move only take 1 turn)
If you want to use this very team then there are a few suggestions that I'd like to give. If you suggest your cousin is using a set up team then you could be very cruel and use the move "topsy turvy" with malamar or smeargle to flip the set up on it's head (if your cousin uses swords dance and you use topsy turvy, then the +2 attack will become -2 attack)
Besides that, i would like to improve the movesets of your team (while keeping them legal):
Groudon-Primal @ Red Orb
Ability: Desolate Land
Tera Type: Fire
EVs: 252 Atk / 4 SpA / 252 Spe
Naive Nature
- Eruption
- Precipice Blades
- Stone Edge
- Heat Crash
Whimsicott @ Focus Sash
Ability: Prankster
Tera Type: Grass
EVs: 252 HP / 182 Def / 4 SpA / 70 SpD
Bold Nature
IVs: 0 Atk
- Taunt
- Moonblast
- Switcheroo
- Encore
Xerneas @ Power Herb
Ability: Fairy Aura
Tera Type: Fairy
EVs: 126 HP / 69 Def / 252 SpA / 63 Spe
Bold Nature
IVs: 0 Atk
- Geomancy
- Dazzling Gleam
- Psyshock
- Protect
Kangaskhan-Mega @ Kangaskhanite
Ability: Parental Bond
Tera Type: Ghost
EVs: 189 HP / 189 Atk / 132 Def
Careful Nature
- Fake Out
- Return
- Earthquake
- Power-Up Punch
Smeargle @ Covert Cloak
Ability: Moody
Tera Type: Normal
EVs: 252 HP / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Baton Pass
- Skill Swap/Topsy turvy
- Protect
- Follow Me
Corviknight @ Red Card
Ability: Mirror Armor
Tera Type: Water/Grass
EVs: 252 HP / 252 Def / 6 SpD
Impish Nature
- Light Screen
- Roost
- Body Press
- U-turn
Note: Kangaskhan lost powerup punch in gen 7
Using the team would be fundamentally same as "The big 6" but it's fine if you don't know about it. To put it simply, You should either try to get groudon in tailwind or try to sweep with geomancy xerneus or baton pass all moody boosts from smeargle to either one of your legendaries.
That is all I really wanted to say
