1. Roll for NP2 at minimum. Your best bet is waiting for Singularity banners where he'll be on Rate Up for Fuyuki. You need this to optimize his damage output since one of his strengths lies in his high NP gain.
2. Get 4th ascension ASAP, as Caster Cu has a reverse S growth curve.
3. Max 2nd skill, then 1st skill, then 3rd skill.
4. Caster Cu is best played as a secondary damage dealer generally speaking, especially in situations where you need to counter multiple class types in a given quest. In short, you can extremely safely play Caster Cu as a counter to enemy Assassins or Zerks without worrying about stray Riders cheesing him due to his overkill durability.
5. Best CEs for Caster Cu are generally things that give NP damage, since it's very easy to charge his NP. Formalcraft is also good, especially if you are stuck at NP1.
6. Caster Cu is flexible in terms of team building due to having a Buster NP, 3 Arts Cards, and a strong self crit modifier. Avoid pairing him with Martha and Marie though, who have much higher star weight than Caster Cu, but aren't notably better damage dealers than him. In general though, you will ideally be playing Caster Cu to complete your team composition, rather than building a team composition specifically around him since many of his strengths like his durability and high NP gain aren't very dependent on his teammates.
7. Personal favorite partners include Hans, Shakespeare, Emiya (Assassin), and the majority of Buster oriented damage dealers (particularly Gilgamesh and Brynhild).