I actually re-read through this thread and found someone who was missing one of the sealed rooms under the sea, but mentioned the forest on a tiny island, so I searched till I found it. And I had about 3 strong weed killers left, which is more than enough
I'm not sure if this strategy was mentioned, but since you can basically spam the break option. A single break with strong weed killer does 50k damage, so the "flower" boss was easily defeated in 2 hits.
I'm kind of hoping for a super secret boss somewhere, but I think that's about it after the side missions.
My plan now it to replay using a new game with the saved data so that I can get (3) of the +5 to everything merit rings.