just an idea, im pretty sure that a defensive core of tyranitar/mega tyranitar, cresselia and alomomola would be a good core.
Tyranitar @ Tyranitarite
Ability: Sandstream
EVs: 252 HP/ 252 Def/ 4 Spdef
Impish nature
- Rock slide/Stone Edge
- Crunch
- Fire punch
- Dragon dance
Cresselia @ Leftovers
Ability: Levitate
EVs: 252 HP/ 252 Def/ 4 Spdef
IVs: 0 Attk
Bold nature
- Psychic
- Ice beam
- Thunder wave
- Moonlight
Alomomola @ Rocky helmet
Ability: Regenerator
EVs: 252 HP/ 252 Def/ 4 Attk
Impish nature
- Waterfall
- Knock off/Toxic
- Wish
- Protect
Tyranitar covers cresselias weakness to dark and ghost while cresselia covers tyranitars ground and fighting weaknesses. Alomomola helps to cover tyranitars weakness to steel. this core can be weak to scizor and pinsir, which is why tyranitar has fire punch(for scizor) and cresselia has T-wave to cripple them.
tyranitars sandstorm isnt much of a hindrance to either cresselia or alomomola due to cresselias moonlight, and alomomolas wish and regenerator.
cresselia helps to beat pokemon that tyranitar cant, like garchomp, who gets 2-shotted by ice beam. fighting types like conkelldur are walled even if they carry knock off as it can simply be healed by moonlight and are killed off by psychic. set up sweepers like mega pinsir are crippled by t-wave.
gliscor is cleanly 2-shotted by ice beam.
every dragon bar hydreigon and zard x is 2-shotted by ice beam.
Alomomola discourages scizor from constantly uturning due to rocky helmet. alomomola is a source of recovery for tyranitar who cannot otherwise heal on its own. alomomola doesnt need to heal itself a lot due to regenerator. knock off is utility for removing items but toxic can be used for chip damage, which works excellently in combination with tyranitars sandstorm.
Tyranitar alone checks both variants of mega charizard. tyranitar can get a free dragon dance off of mega charizard X who can only do 45% max to mega tyranitar with its strongest move, outrage.
mega tyranitar checks charizard y due to the typing advantage and sandstorm having priority due to tyranitar being slower, but must watch out for focus blast, which usually wont kill if zard y is timid, but has a 68% chance to OHKO, if it is modest.
fire punch is for scizor and ferrethorn. the Evs in this set make it so t-tar isnt 2HKOd by mega scizors bullet punch.
the EVs in the set allow tyranitar to live one close combat from mega pinsir, jolly and adamant sets and OHKO with rock slide/stone edge.
tyranitar can comfortably take out most dragons bar garchomp due to their common secondary flying type.