As far as I can tell this is for killing the plant boss at the end of Act 4: Chapter 2. In theory it's story related and can not be missed.
That being said it did not pop for me when I went through on my normal play through. It only popped when I went back through and beat the boss on very easy. No other guides say you need to do anything specific, though on my normal playthrough I died a bunch where on my very easy I beat the boss without dying.
If you don't get it the first time you can chapter select back and try again.