I too have been working on the achievement. Only things left to log are 2 plants, 4 enchantments and the rotberry dart. I was struggling to find a phantom piranha but found one yesterday.
That's quite an achievement! It's really tedious to fill it all in.
There's only one entry for which I have no idea what it is. I'd prefer a hint rather than an outright spoiler.
It's the same silhouette as the blandfruit plant, but it's something different. Can't say I've ever seen anything that matches that description.
Now that I think about it, I don't think I've ever tried planting the Seed of Rotberry. Nor have I made a Rotberry Dart, but I know that's the dart I'm missing. Makes sense that it would be whatever plant the seed makes. The thought of doing runs to unlock these without getting the free wand hurts me.
I figured it out, of course, but what were you hinting at? That the Rotheart has the same silhouette? It's indeed similar, but it's not the same. Now, the bush that the seed makes is the exact silhouette of the blandfruit bush.