First and foremost, getting past the pawns' Focus Sash ability and preventing Trick Room from going off will make things much smoother. An easy way to do it is either using Fake Out to break the sash effect or using a multi hit move, preferably super-effective ones. His mons except Metagross have weaker physical defense, so physical attackers are ideal. They're also slow due to expecting trick room.
For mons, you can use a strong bug type like Scolipede or Beedrill that get multihit bug moves (Pin Missile for power and Twinneedle for reliability) . Medicham, Hitmonchan, and Hitmontop get a variety or strong moves boosted by abilities despite being Fighting types. Ambipom gets both Fake Out and a variety of technician-boosted moves, including Beat up and Feint Attack (further boosted by the terrain). Bruxish and Tsareena both are physical attackers with abilities that block all of Metagross's priority attacks. As with all Double Battles, aoe attacks are great, especially combos like Manetric and Raichu boosting each other with Discharge and Lightning Rod.
For items, Silverpowder can be found on wild Mothims and Wormmadams (both rare Headbutting trees in North Aventurine Woods. Black Glasses are found on wild Sandiles in Tanzan Mountain. Spell Tags from wild Shuppets, Duskulls, and Yammasks in the Abandoned Power Plant. Gems also work, as well as Plates if you're patient to soft reset on mining rocks. If you use special attackers. Synthetic Seeds give a free Special Attack boost.
Working around the boss' strategies or beating them at their own games is how to win.