This battle is pretty straight forward, Faiz is mostly a ranged boss, relying on poison to cripple you. He has several, avoidable, attacks (AFAIK, they don't have names, so I'll be winging it).
- Tracking Poison Bomb - a Giant, single tracking ball of poison. This will pass through all obstructions, and will follow you until it hits you or is deflected.
- Eyeballs - Several eyeballs that launch at you, easy to dodge, just run/roll sideways.
- Poison Rain - Not actual rain, but dozens of projectiles that DO NOT go through objects, and have a limited range. Hide behind something or move far from Faiz to avoid them.
- Poison Flowers - Flowers pop up and explode, dealing poison, simply keep moving to avoid.
- Slugs - Slugs fall from the sky and attack the player, exploding, dealing poison. Attack or block and roll away.
- Scythe - He also will warp to and away from you to attack physically, these attacks can be deflected, and gives you an opening to attack.
- Feather Storm - After you attack him at close range, he will surround himself with feathers that stun you if you get caught in them. This leaves you open to any attack from slugs, or poison.
- Poison Cloud - When he reaches 1/2 health, he will teleport to the pillar and let loose a cloud of poison, go to the far side of the arena to avoid this, be warned, he will use the above spells as well.
- Hex/Curse - Only happens once during the second half, 4 small summons will begin casting an instant death spell, you must kill them all before the bar fills. Luckily, they spawn around the pillar in the middle, and only take one hit to kill.
If you can properly time your deflects, this fight is easy. Use your items as well, including the Firebombs, as they drain his health over time. Spears deal a decent amount as well, but become less reliable as his health drops, as he teleports more quickly, dodging them.
If you get poisoned, stand in the water and it will cause the poison to drain faster, and also net you an achievement.