Looking at the Bel-chan event, I think it might be best off to just grind 4-1/2/3. Easy to 1:1 clear quickly with just about any low oil ship, and only 4 battles per stage for 80 points, and you'd still get purple plates. It will take 125 clears x 4 battles each = 500 battles total, but that'd be faster and cheaper than the higher level chapters. For instance, you'd need 72 chapter 10 clears x 7 battles each = 504 battles total, and those battles will be longer and far higher oil cost. Any way you slice it though, it's a big grind. Like 8+ hours worth of grinding.
This wouldn't be bad if it were the only event for the time period, but seeing as how it's mutually exclusive with the Kizuna event, it's rather shitty. We also have less time to do it than the other regions. We're on the rerun version where you get no points for daily tasks, but we have the time span of the original version where there *were* daily tasks that gave out large chunks of manjuu. The rerun version should run a month. I think they didn't properly plan this out properly. I expect a time extension or it altered so the SP maps count for manjuu.
Edit: Purple plates are ch 6+, so probably best to do 6-1 on repeat instead.