The event is easy to get 9-10 pieces and a complete (or close enough) level 2 set. Annoying but not hard and with lots of time leeway (like 10 days worth).
If you work your ass off and no-life it you can get to 11-12 by running cities as tight as you can and skipping the right tasks.
13+ takes diamonds no matter what (and diamond spenders have to work hard to boot if they want to increase mileage of those diamonds
)
It's very easy to do "ok" - just don't do anything dumb (extraneous manager levels, time skips, buying 2-5 chests or 7+ at stage end (1 is free, 6 is an efficient-enough set kit, 2-5 is wasting pots of gold for no progress, 7+ requires you to spend an additional 3900 for a kit instead of 1900 for a kit; they really need to communicate the 1900 pots of gold at stage end better - i.e. rather than showing all 6 as free perhaps they should grey out 5 of them and show the prices they *will* be)
It's not at all rewarding to try and do better than ok - way more work for little more reward.