Cu Chulainn (Caster)

Cu’s kit at endgame revolves around At the Lake. By sacrificing himself after the turn and gaining 80% NP bar, he is capable of repeatedly using his Noble Phantasm. Primordial Rune is used in his initial charge, or a later one depending on the supports a Master is using. Protection from Arrow primarily gets used at the start of a battle just in case Cu does suffer damage, in which case it is vitally important as Cu will likely only have 3000 HP from Guts revives to live on for most of the battle.

• Rune Spell/Primordial Rune is one of Cu’s two charge, providing up to a 30% gauge charge. While its Debuff Resistance and Crit Damage bonuses are nice, they don’t often find use with him. Leveling this will depend entirely on if a Master has both Vitch and Oberon, or if they are lacking Oberon (or both). If a Master does not have both Servants, it should be leveled to max to take advantage of the 5-turn cooldown and full charge potential. If a Master has both, this skill can be effectively ignored in leveling by using both Koyanskaya charges turn 1, this skill turn 2, and Oberon’s charges turn 3. Either way, it should be leveled second.
• Protection from Arrows makes Cu much harder to kill. While it is a powerful skill with its three-time unlimited evade and Defense boosts, it has poor scaling (and primarily scales Defense). This skill should be leveled last and may be best ignored in leveling. 
• Disengage/At the Lake is Cu’s signature skill. It initially is just a debuff clear and a small heal, but after Lostbelt 6 it becomes a powerful charging skill. Cu will gain a powerful attack boost which stacks, along with a delayed debuff. At the end of the turn, he will die and be reborn with a self-sacrifice and Guts, giving him 80% NP charge. Its low 5-turn cooldown is what enables Cu to so effectively loop Buster damage. It should be raised to max to reach the 5-turn cooldown, and raised first.

“Hey hey, if you make me too strong you won't be able to control me.”

For most of the story, and for new Masters, Cu Caster will seem like a vanilla and forgettable Servant. However, once a Master clears Lostbelt 6, he transforms into one of the premiere AoE Buster Damage Dealers in the game!

What truly transforms Cu from a vanilla Servant into a powerful one is his NP Charge skills, specifically Primordial Rune and At the Lake EX. Due to the massive delayed charge of At the Lake, and its short cooldown, it is possible to fully charge Cu every turn with room to spare given the right setup. In addition, he has rather decent damage in the right teams. Also, in common with other Cu counterparts, he is rather durable with Protection from Arrows A’s evasion and defense boosting capacities.

Cu does suffer some from his vanilla Servant status straight from launch. For one, his non-skill related NP gain is atrocious, leaving him much weaker until Masters clear Lostbelt 6. He also suffers from low natural attack, making him entirely reliant on supports and CEs for damage boosts. In addition, he cannot effectively use his Critical bonus due to having no real way of gaining Stars, making him NP-reliant.

Cu Caster is not particularly well used until after clearing Lostbelt 6, and even then only with the proper supports. But if a Master has the proper supports and has cleared the story to unlock his skill upgrades, Cu will transform into one of the best AoE Casters in the game.

Dying

As silly as it sounds, Cu’s biggest strength comes from repeatedly dying with At the Lake. This will create and trigger a Guts status with a self-sacrifice, charging his NP gauge by 80% on a low cooldown. By using proper supports, he can repeatedly die and charge his NP gauge allowing for easy looping conditions. 

Not Dying

Cu is, at his heart, a Cu. With the ability to clear debuffs, Protections from Arrows’s no-turn-limit evasion, defense boosts, and Debuff resistance, he does not go down easily. He’s more likely to kill himself than be killed. 

High Damage AoE Buster Looping

Cu has so much NP charge that he is capable of AoE Buster looping with a Black Grail. With the bonuses from supports, this can result in huge damage potential, among the best in the game. When combined with the power of Oberon, he will obliterate nearly all Assassin enemies with no reliance on refund. 

Natural NP Gain

Cu is a launch Servant, meaning he is plagued by low hit counts. With only a 2-hit Quick and 3-hit Extra, he will struggle to refund his gauge without his third skill. 

Support Reliant

Cu without the proper supports will be unable to effectively loop with his Noble Phantasm, and will find his damage significantly drop. 

Cu’s true potential comes with The Black Grail CE. Without one, his damage will drop off dramatically, but he still will benefit more from those that boost NP Damage. In rare situations where 4 Noble Phantasms are needed, he could also use Starting NP Gauge CEs to get another one off, though at significantly less damage potential. As he does not require a charge, he also is a prime candidate for Event Bonus CEs.

• The Black Grail: Cu lives for this CE, using its 80% bonus to significantly ramp up his otherwise low damage. If Oberon is involved, this CE will have its strength multiplied, creating a true monster.
• Heaven’s Feel / Bitter and Sweet / Angel’s Song: The more NP damage Cu has, the better. Bitter and Sweet also comes with a bonus of two invincibility hits, helping him survive for longer. 
• Kaleidoscope / The Imaginary Element / Dragon’s Meridian: At the cost of damage, if Cu has a starting gauge he can potentially use his Noble Phantasm for a fourth turn in a row with the proper supports.
• Event CEs: As Cu is not reliant on starting charge or refund, he can often hold Event CEs so long as his damage is sufficient.