My 1st build which is able to farm Mayhem 4 effectively
Shields (prefer Transfomer for better survival): ASE 50% bonus elemental damage
Grenade: ASE 50% bonus elemental damage (different element with shield)
Class mod: Phasezerker with Clarity for health regen (or Conflux for more ele dmg ?)
Artifact: Pearl of Knowledge
Weapon (prefer Maliwan due to the skill "From Rest"): ASE 100% increased weapon damage or next two magazines have 50% bonus elemental damage.