Geo Xern + Z Duggy
sorry i couldn't be bothered to find a dugtrio sprite, use your imagination
Xerneas @ Power Herb  
Ability: Fairy Aura  
EVs: 108 Def / 252 SpA / 148 Spe  
Modest Nature  
IVs: 0 Atk  
- Geomancy  
- Moonblast  
- Focus Blast  
Dugtrio @ Groundium Z  
Ability: Arena Trap  
EVs: 252 Atk / 4 SpD / 252 Spe  
Jolly Nature  
- Earthquake  
This core is just about always able to break through teams and this case is no exception. Though this team does have priority which can usually stop Xern after it sets up, if you setup on waterceus and either trap or chip  pdon (also hit the fmiss on steelceus which is about a 9% chance to happen) then the game will end
some side notes 
- 148 speed xern at +2 outspeeds scarf mray
- dug can also trap the steelceus i guess but it's a speed tie so l o l
- even if this fails and you're not able to trap pdon, geo xern is still a massive threat to the team and can take out multiple mons 
calcs
252 Atk Life Orb Technician Marshadow Shadow Sneak vs. 0 HP / 108 Def Xerneas: 117-138 (29.7 - 35.1%)
252+ Atk Arceus Extreme Speed vs. 0 HP / 108 Def Xerneas: 127-150 (32.3 - 38.1%) 
0 SpA Arceus-Water Ice Beam vs. 0 HP / 0 SpD Xerneas: 77-91 (19.5 - 23.1%) 
0 Atk Groudon-Primal Precipice Blades vs. 0 HP / 108 Def Xerneas: 202-238 (51.3 - 60.5%)
252 Atk Rayquaza-Mega Dragon Ascent vs. 0 HP / 108 Def Xerneas: 234-276 (59.5 - 70.2%)