Um... OP, there are three ways to get the mochi in the fate:
1) Kill opo opos
2) Kill the mochi monsters
3) Run in to the mochi tub, interact with it to grab some, and run away.
ANY class can do these things. What you are supposed to do is focus target the thing with the hammer so you can see it casting. Some of the casts are super fast, some are super slow. So you wait until he's doing a slow cast, then you can just run in and interact with the tub. If you only kill opo opos from far away, it takes forever...
On a slow cast, you can grab from the mochi tub like three or four times in one go.
And I love the housing objects! I mean, it would have been nice to have a different fate per area and have to do it 3 times for different items or something, but the idea of it was decent.