Here's some strategies that can work. I'll cover items and abilities, and then give teams that can use them.
Items
Choice Items:
Choice Band and Choice Specs are viable, as they can give Pokemon an immediate boost to an attacking stat. Some abilities can be used to be used with Choice items to make attacks more deadly, such as Defiant, Competitive, Intrepid Sword, and Download.
Weakness Policy: Weakness Policy is a good option, as Metronome can summon super effective attacks that can let you boost your attack and special attack. This works well on bulky Pokemon.
Eviolite: This item is just for Pokemon who aren't fully evolved, such as Dusclops and Type: Null. This item is used to give the NFE Pokemon a boost for their defense and special defense. This is used with Friend Guard so the NFE Pokemon can take as little damage as possible.
Abilities
Intrepid Sword: Gives you an attack boost at the start of the battle. Works well with Choice Band, epesically on a Pokemon with a high attack stat such as Mega Heracross.
Flower Veil: This ability really good for Grass-types like Mega Venusaur and Necturna. Flower Veil prevents status moves and stat drops on Grass-types, which is really nice for them.
Friend Guard: This is for the Eviolite users I mentioned. This helps said Pokemon take as little damage as possible.
Primordial Sea/Desolate Land: Primordial Sea/Desolate Land is used to eliminate Fire and Water moves, which is useful for Pokemon such as Mega Camerupt and Mega Abomasnow. Pokemon such as Regirock can use Desolate Land. Pokemon such as Mega Abomasnow and Glastrier can use Primordial Sea.
Simple: This ability is a double edged sword. While Simple lets you get double the positive boosts, you also get double the negative boosts.
Magic Bounce: Useful for bouncing back status moves that could cripple you
Teams
Double Intrepid Sword
Flower Veil Fun
Bonk's Stall
Desolate Land (For this team you can experiment with Analytic on Mega Camerupt with 0 speed IVs since Mega Camerupt is really slow.)
Primordial Sea
So, yeah. That's all I have to share for now. Good luck and have fun!