Our DM originally began by targeting two characters, the Sorlock and the Bladelock, by giving enemies Brooches of Shielding to nerf them. Realizing instead, he had just given us a new revenue stream, by killing these enemies and selling the brooches (naturally, after any party member who wanted one, got one). He did give one Boss a Mantle of Spell Resistance, now the Wizard wears it. He greatly regrets this choice. Our party averages around 9th level. Now the whining has changed to "most of the party are doing too much damage".
He has recently started giving all Bosses, and their assistants, Potions of Spell Resistance & Potions of Invulnerability that they drink immediately before the initiative roll. Now, everyone above a summoned peon, have both advantage on saves and halving all damage against them. Other than the Heightened Spell Metamagic option, which only the Sorlock has, is there any way to bypass this "nerfing" of our party?