This achievement is not that hard. You have to defeat the final boss to unlock it. It doesn't matter on wich difficulty you play. Easy is enough.
To get every medal in the game you have to beat this boss on the hardest difficulty. Someone did find out good equipment to defeat Malvedor on Deathbringer (hardest) difficulty:
Weapon: Dark Angel - Ultimate form of Great Axe
Shield: Tiger´s Claw - Ultimate form of Spiked Shield
Headgear: Skullhelm - Ultimate form of Totemic Helm
Armor: Epic Spiked Armor
Ring: Serpentine - Ultimate form of Swamp ring
Necklage: Iriki´s Fang
Boots: Epic Miner´s Boots
Belt: Epic Belt of Law
Thanks for the guy who made this armor list!