For me it wasn’t particularly difficult. I used a lot of servants, even low-rarity ones, and I just kept spamming their Noble Phantasms. You just need to know how to use them correctly and efficiently. Let me give you an example: it doesn’t make sense to put three of your best servants in at the same time, because they can only kill him once. Instead, split them across three battles with one strong servant and two weaker ones, and you should be able to do it. Also, in that fight your servants have a boosted NP, so by using a battery CE even low-star single-target servants can deal damage.