Arash

Arash, the legendary archer of Iranian mythology, is one of the best Servants for farming and clearing waves. His Noble Phantasm is both easy to level up to 5 and insanely powerful to boot. As an Area of Effect NP, it can reliably kill a wave of enemies in daily missions (with the exception of Lancer enemies) in one go, with the caveat of Arash dying upon using it. A team fielding Arash usually has him started with a CE which increases starting NP charge, along with support Servants who can charge his NP as well, so that he can fire it off as soon as possible.

Though seemingly detrimental, Arash’s death upon NP use can actually be useful as it allows for another Servant to replace him immediately. His death can even be used in a combo with certain CEs that have an effect on death. However, apart from his very strong NP, Arash lacks the stats and the skills to impact battles. As such, it’s recommended that he keeps to his farming/burst damage niche. All in all, Arash is definitely a Servant that should be raised even just for his sheer usefulness in farming.

One of the Best Servants for Farming

Arash’s AoE NP can be used to clear almost any wave (except Lancers) during any of the daily ember/QP quests, while killing him in the process. However, the fact that Arash dies can actually be a boon as it allows another Servant from your backline to instantly replace him without having to use the Order Change ability from the Chaldea Combat Uniform. Additionally, as he is a 1-Star Servant, he only takes up 3 cost from the party. As a result, a Master can place other more higher rarity Servants in their farming team (for bond point farming).

Death upon NP Use Combo Potential

Outside of farming, Arash can also be used in normal battles as a big damage nuke followed by another Servant replacing him. This allows for some nice combos with Craft Essences like 500-Year Obsession which gives beneficial effects for the team upon death, as well as immediately getting another Servant out to battle.

Limited Use Outside of Niche

Having low star generation, stats and a vanilla skill set make Arash less than ideal for battles outside of his niche. He is best used as a one-time high damage burst followed by other strategies. Thus, building a team around Arash is not recommended.

Death upon NP Use Detrimental in Tougher Battles

Arash’s NP is a double-edged sword. Aside from being able to deal a high damage nuke and his resulting death being able to combo into other strategies, using Arash in such a manner would still leave you one Servant down overall. Thus, a Master should be careful when using Arash during tougher battles. Note that when Arash uses his NP while he is the last Servant standing, the battle will be considered a loss even if the enemy perished.

• Arrow Construction A is a very useful skill to help Arash charge his NP, enabling him to use it immediately with a non-LB’d Kaleidoscope or at higher skill levels, a LB The Imaginary Element.

• Toughness EX increases Arash's tankiness, but it doesn’t really affect his playstyle that much considering his NP is usually charged up and used as soon as possible. Arash isn’t meant to be tanking hits. 

• Clairvoyance C increases Arash’s star generation, but given Arash’s low hit counts and single Quick card, he can’t use it to properly generate stars. Unless materials are plentiful, this skill can be safely avoided.

• Kaleidoscope / Imaginary Element: The definitive CE equip for Arash. It allows for instant NPs with Arrow Construction for quick battles. The faster Arash can fire off his NP, the better. 
• Devilish Bodhisattva / Golden Sumo / Holy Night Supper: Sometimes Arash needs a slightly bigger boost to his damage to fully clear a wave. Combined with Servants like Zhuge Liang, Arash can still fire off his NP instantly despite the reduced Starting NP Gauge. Arash scales very well with Overcharge, making Devilish Bodhisattva an excellent option. 
• Sacred Devotion / 500-Year Obsession / The Merciless One: If there already are other Servants like Zhuge Liang who can charge Arash’s NP, having a CE which gives beneficial effects upon death can be useful in battles against tough enemies. Arash's Bond CE is particularly strong for this purpose.