Diablo III

To earn the achievement "The Lord Of Lies (Inferno), you must confront and defeat Belial at the end of Act II on Inferno difficulty.

To unlock Inferno mode, you must complete the game on Normal, Nightmare, and Hell modes first. I should also mention that you need to be at level 60 before killing Diablo on Hell difficulty (extra credit goes to General Joury for mentioning this in the comments!).

Once that's done, play the game on Inferno mode until you get to Act II and make your way to Belial, who serves as the final boss for Act II. Make sure you have your best gear equipped and your chosen skills ready to go before dealing with him. If you want to make this achievement go by quicker, just set the difficulty to "Easy" (unless you want to challenge yourself on a higher difficulty).

Once the boss fight begins, the first phase is dealing with Belial's serpent servants. Using your best gear and skills, destroy them all. Once that's done, the second phase of the fight begins.

Belial will appear in his first form in the second phase. He'll try to damage you with his magic, but as long as you have healing spells or items at your disposal, it shouldn't be a problem whittling his energy away until it reaches near the end of his lifebar. He'll teleport to the center and transform into his final form and that begins phase three.

In his final form, he'll try to hit you with his pincers or use his bad breath. He'll even try to summon plague portals to take your life energy away. Just make sure to defend yourself wisely by rolling out of the way or using skills such as mantras to keep your defense up. Just keep on hitting him with your best weapons and skills, such as Earthquake, and he'll eventually go down without much of a problem, thus netting you the achievement.

Also, if it's possible, make sure to kill him quickly before he activates his Bad Breath attack. If you accomplish that, you'll complete the Bad Breath challenge, which can only be done on Inferno.

Finally, you don't have to tackle him alone on Inferno. To make this achievement go faster, make sure to bring some friends with you for the ride, especially those who haven't earn this achievement or the challenges yet!

Have fun and good luck!