Help creating a good team

This is my first time trying to help someone make a team, so excuse me if I make a bad one. From what I am seeing, I would say you want to let Cu alter shine. This means you may want a team that centers around him. That is a buster or buster crit team. If you want a regular buster team, Heracles could go with him, having 2 very survivable beserkers together. The one most important thing you might want to try to do is to keep the beserkers alive. To do this, you might want to add Irisviel for her aoe guts, David for his aoe evade, Mash, and maybe Martha (not really suggested) for her aoe heal. To increase the damage output, which is not too necessary considering that your team is doing a lot of damage with two beserkers, you may want to prioritize David again for his charisma. Because beserkers have a super low star absorption, critical stars will not do too much, but you can always have a ce that does that, you can add maybe Lancelot plus kaleidoscope, Hans, your Assassins, and maybe Mozart after the rank up quest.. For craft essences, limited zero over or verdant sound of destruction for buster up, kaleidoscope and the imaginary element for starting np, 2030 for stars, and a star asorption ce if you want a buster crit team.

In summary:
Heracles, Cu alter, David or Cu alter, David, and Mash.

I hope I did this well as after all this is my first time.

this is also my first time trying to help someone in the game so forgive me if what i say may not be the BEST team possible.

i'd agree with Bobina that you might wanna build your team around cu alter since he's a really good berserker, and durable at that. maybe start off with cu alter, chevalier, and waver (supp). this will probably be a guaranteed 3turn survival for waver and cu alter since chev will be tanking. during first turn you should already activate cu's dodge in advance even when chev is tanking so that the cd will already start ticking, while still having 2 dodges come turn 4.

after chev dies, mash should ideally be the replacement. she can taunt, give def up, and invincibility so that will probavly buy you another 2 turns guaranteed survival of cu alter, take note that by this time cu alter should still have his 2 dodges up and if you have maxed out his dodge skill, the next turn would have it fully cded and ready to use in case the 2 dodges get used up, effectively giving your cu alter 4 successive dodges. more than enough for about 85perc of boss fights.

for the last 2 servants, maybe have your cu lancer 5th to stall even more and just drain the enemy's skills, nps, and health bar. for last servany, herc might be a good clutch damage dealer since he can have a guaranteed 2 turn survival, 3 turn if you already got his max bond ce castle of snow.

last for the strategy part, during the time when other servants are tanking for cu alter and waver, you should aim to either deal the maximum amount of damage, or charge cu alter and waver's np, so for this you should generally avoid using quick cards as much as possible.

for ces maybe zero over or verdant sound of destruction on cu alter, iron willed training on chev and mash, anything on cu lancer (depending on your preference, but maybe ox demon king to give a boost to cu alter for 3 turns after he enters), and for herc castle of snow or if you dont have it, same as cu alter zero over or verdant.

and... thanks for reading, if you did. hehe

For general purposes, Cu Lancer or Heracles will be your primary damage dealer, and will need supports like Waver, Mash, Georgios, or Caesar/Shakespeare to do well long-term. The first three names there are to help keep them alive (your David helps, but not much past a single dodge), while Caesar/Shakespeare helps them pump out damage.

Alternatively, you have Shiki. Put her on a team with Mash/Hans and a friend's Waver, and you've got yourself a single target Death machine. Numbers wise, it's not nearly as much as Cu Lancer or Herc, but you'll be spamming an insanely high-% auto-Death every turn or two with all those Arts cards, plus spamming the defensive NPs of Mash or Hans. She also generates a large number of crit stars, and you'd be surprised how much damage a crit from a friend's Waver can do. The real downside is that she really depends on Mash or Waver to stay healthy, since she only has one dodge for herself. (side note: I really want to try using her with Mozart post-strengthening)