Rule #1: Never have more than 1 mega evolution.
Typhlosion
Mega Typhlosion should always be Timid. Since Hidden Power Grass is hard to optain, I don’t recommend it often. If you are looking for a good fire type, you can ask for a Delta Greninja, Chandelure, Delta Chandelure or Speed Boost Blaziken.
Giratina
It’s a legendary Pokémon, but it is not that good against the elite 4. Still, it is very usable if played correctly and if you want it, use this set:
Giratina @ Leftovers
Ability: Pressure
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
-Shadow Claw
-Dragon Claw
-Iron Head
-Draco Jet
Draco Jet will be used to revenge kill the champion’s faster dragon types (#no spoil)
Delta Gardevoir
You either use it as a Choice Scarf or as your ONLY Mega. Ice types are generally very good against the Elite 4 and a Choice Scaf set is especially good against the Champion.
Delta Gardevoir @ Choice Scarf
Ability: Lighting Rod
EVs: 252 SpA / 4 SpD / 252 Spe
Quirky / Timid Nature
-Thunderbolt
-Ice Beam
-Moonblast
-Earth Power
Thundurus
I think this one is not necessary. Use something else. My first advice against the Elite 4: Bring a taunt user. It 100% shuts down Kyla and if your taunt user is levitating, it also shuts down Eduard’s lead. Good taunt user in this case are Mew who can battle pass on its own, Gengar and Gyarados. To avoid sharing Ghost weakness within your team, I suggest you drop Giratina if you use Gengar.
Mew
I would go all out on the supportive set, personally. I would use a set of:
Mew @ Leftovers
Ability: Synchronise
EVs: 252 Hp / 4 SpA / 252 Spe
Carefull Nature
-Psychic/Spyshock
-Baton Pass
-Swords Dance/Nasty Plot
-Rock Pollish
Even if I think there are better options than Mew.
Delta Venusaur
I really recommend it as your mega. With amazing bulk, it is a great bet in a lot of situation. If you use another Mega, then just don’t use Delta Venusaur.
Contrairy of A LOT of the community, I recommend using Draining Kiss on bulky variant as it will ofter take a hit before attacking.
My remplacement ideas
Taunt: Gyarados, Gengar
Generally good Pokémon: Scizor, Choice Scarf Mold Breaker Excadrill, Tyranitar+Sand Rush Excadrill, Delta Haxorus.