Prisma Causeway event

Good question. I wish I knew more about this event, but the JP wiki just gives battle information, not detailed run-downs of how everything works.

I would say hurry and raise ushi and maybe cu but mostly ushi. Also im surprised you haven't alrdy finished camelot before the anni (its clearable before the anni without apples). lastly while your servants might not do that well in event, supports should absolutely be able to carry you as long as you have good supports.

Personally, I'm going to wait for the event to start before leveling anyone and I would suggest the same. Why? Because first, it's not like those embers are going anywhere. Second, you need them to ascend Kuro(minimum of 135, 315 if you wanna max level). Third, cause I'm not sure how much Kuro is gonna contribute to the event. She's got an inherent +100% atk boost and the event CE gives her another 50% atk and arts card(read NP) up. She will be hitting hard. You will be getting a permanent copy after you clear the main quest. So, I say stock up on embers and wait.
Also, Ushi's got a quick NP which doesn't get a boost. Don't see why we can't just use Kintoki.

Where did you read it is a difficult event? It really isn't.

I've mentioned it before in a Question thread, but I played this event on JP as my very first foray into the game. I started a few days in and went in blind and without understanding the game/Japanese.

Before the event finished, I had completed every single facet of the event. All missions were cleared and I even managed to defeat the final quest for a Crystallized Lore.

The Raid HP numbers seem a lot more daunting than they actually are. You have nothing to fear.

I have heard Ushi is the go-to servant for Prisma Causeway due to the abundance of caster bosses. I would also recommend leveling St. George because his tankiness and 3-turn taunt is phenomenal against raid bosses.

The biggest issue with the Illya event is that it's a similar format as the Fate/Zero Accel event, in that you have a bunch of missions you need to complete and therefore you need to devote a lot of AP to its completion (and there's no BP for the boss fights like in Onigashima). My advice is save your apples for the actual event, unlike me who used every last apple to grind out ascension mats from the summer event.