From what I've seen from alliance chats:
The thing where you can't get 100% success rate feels unfair for the amount of effort the event requires
Especially later on where there's more chance to fail even with perfect ratios
This is a little thought I had
The base chance lowers throughout the boxes, but the chance you get for finding the right ratio increases accordingly so that the total is always 100%
The next change isn't necessary but I can imagine that Inno would want there to be at least something that you have to do to get 100% chance
Every mission requires two types of units, just like now, but the ratio is randomized
To help with getting the ratio we get the value we need to beat (for example x ranged attack y sharp attack for an escort quest)
That way we can figure out the right amount of units to send to have that same ratio but in the corresponding defense, maybe dumb the units down a bit so that archers don't have ranged def and such to make it easier for people
Basically the same event overall but less infuriating