How do I beat the Lord of Lies (Act 2 boss)?

You can try crafting/farming some gear that will better optimize your character. Chances are you are not doing damage fast enough as the poison on the ground spell acts as a "soft enrage" in that the longer the fight goes, the more ground area it will cover.

As suggested in the comments you may also need to either level up your character some more or drop the difficulty down.

If this isn't your first max level you can also do bounties to get gear and level up and then come back to him (although I believe he does scale to your level).

You can also bring a friend into your game who is higher level to help kill him. Since you are the creator of the game, Belial is spawned at your level and your higher level friend will have an easier time killing him for you.

DO NOT LEVEL. Leveling will not make the boss easier. This is not a static game where you can 'over-level' in order to kill the boss fast, instead your gear will be weaker and the boss will scale to your level. Dropping the difficulty will work wonders; grinding levels will hurt you. Gambling will always get you items scaled to your level. Once again DO NOT LEVEL, unless you're just going to go to 70 and optimize. (Keep in mind you can access adventure mode without beating the plot.) Making sure your items are all within a few levels of your character will help.

An easy way to bump damage is to get a higher end gem from another character ( hop into a friend's game and grab a high level gem drop) one imperial ruby will up your damage sufficiently to kill him readily. And they are not level tied so you can use one on a level 5 weapon with a socket ( use a level 33 for your case).

@WizLiz already covered where to stand and how to keep moving to avoid the poison pools. Sadly you people using controllers instead of a mouse will have issues with precision. Keeping moving is the most important once the pools are being spawned constantly. It comes in waves, there is a definitive pattern to his attacks and knowing when to hide will help.

Or lastly just get a level 70 to hop in your game and do the work for you. Even if you're dead, you will get credit for the kill in terms of the campaign and can even earn the trophies associated with dodging every attack.

And once more, DO NOT LEVEL unless you're just grinding to the cap. Yes, a few levels might get you a fancy new skill to try out, but it will also up the bosses stats. If you need some help drop me a line here on SE and we can exchange gamer tags, I am on Battle.net most nights.