Phantom Dust

First you need 10,000 credits to buy a 1-School Arsenal Case. You can earn this easily by playing the SP game normally or repeating missions on the Memory Box Terminal in the Visions HQ room at the start of the game.

Buy the Arsenal Case,

Multiplayer Mode>Xbox Live>Shop>Buy Arsenal Case. Then buy the case for 10,000 credits.

Setting up case,

Multiplayer Mode>Xbox Live>Lab>Edit Arsenal. Select the one skill case (Limit[1]). Open it and it will be full of Aura Particles. Select an Aura Particle and replace it with a Faith skill. Faith skills are the skills with a eye symbol (not the pyramid one).
If you are playing a real match you'll need to build proper arsenal, not just add one skill for boosting purposes.

If you need to buy a Faith skill,

Multiplayer Mode>Xbox Live>Shop>Buy Skill. Attack tab, scroll to the bottom and you are looking for the eye symbol (not the pyramid one). Buy Hell's Fire for 50 credits (or any other Faith skill).

Complete list of skills here - http://phantomdusted.com/pd_skills.htm

To win the achievement you have to select the single-school Faith arsenal case when setting up either a Xbox Live match or a split screen match. The opponent can't just quit, you either have to beat him or he has to kill himself.

This can be completed in split-screen or online, the only requirement is building a deck using a 1-school arsenal and winning a match with it.

I know it seems obvious but there was confusion on what modes this could be earned in.