3BP event team help!

So far I’ve done well on the first two, so I decided to do the 3BP one and out of my two attempts so far, I’ve only completed one attempt. Narrowly at that.
For my current set up I’m using:
Cu(Lancer) lvl 70
Cu Alter lvl 100
Cu Alter lvl 98 as my support
Proto Cu lvl 70
David lvl 70
And Mash lvl 60

I used all my Cus(except caster) because of their high survivability and heavy damage(especially those double Cu Alters) David and Mash in my head seemed like good support thanks to David’s evade skill and Mash’s taunt, but my problem comes when they start dying off around 8-10 turns in and then I have to somehow manage not letting my remaining servants die off. I’ve been using the Atlas mystic code for its skill cool down ability but I’ve been debating to replace it with the battle mystic code. Or is my team the problem here?

My other maxed out servants are:
Brynhildr
Siegfried
Shiki(Assassin)
Lancelot
Irisviel
Kid Gil
Diarmuid
St George
Hans
Hassan
And of course,
Angra Mainyu