The Triffid is where you will find Green Goo to make Creeper Repellents and the Big Bertha. But you can't hurt it when it's closed up! Be patient and wait for it to open until you try to hit it. Meanwhile, be careful not to fall or get knocked off the round island that it spawns on. Perhaps making a backstop behind you would be useful? Found in the Danger Dimension.
Can't be hit when closed.
Knocking surprised players off islands
How to Kill
Be patient. Wait for it to open.
Found: circular islands in Danger Dimension
Drops: Green Goo