How long is it going to take me to get a sacro breastplate on VD?
Can reasonably get it on D as well.
Drop rate is much better than Malignance and the BC fight is, I'd say, much easier than Lilith (altough this kinda depends on your setup, if you have mules etc).
Personally I farmed D completely solo (just trusts, make sure to pick the right ones) and got all Alexander drops pretty fast in a very relaxed way.
Edit:
Back when I farmed it I was on THF using Evisceration with Tauret (Rudra's Storm applies a debuff so it's a no-no) and Cornelia was available, I guess that was a big change in terms of kill speed?
When choosing trusts keep in mind you have to avoid those that land debuffs on the boss and those who use WSs that have a built-in debuff or that can make SC with your WSs.
So for instance avoid RDM trusts, avoid Joachim, but can use Ulmia, Ygnas, Monberaux and GEO trusts like Kuyin Hathdenna, Brygid and so on.
I don't remember if Sylvie's entrust debuff (Indi-Frailty) procced Alexander or not, maybe someone will add up on that.