How do I kill those plants for good. I ran into them on the way to Shu Castle. I managed to kill 'em both by standing across the moat and Flamestriking them from a (large) distance, but I ran into 'em again when I covered that ground later. Please help me get rid of that vermin!

There are 4 of them. (that I know of) I killed them the same way, from a distance.

I don't know how many there are but once gone mine did not return. You have the right battle tactic. Note: if you take the high ground they have a bit of trouble up hill. An occasional flame drop will "help" hold them in place.

when you first explore that area you can go past the burnt up house that the rat dude is by and go north from there, and somewhere arounf there is a narrow gap in a forest that you can lure all four of them into that they can't get through, then you just sit back out of their range and cast flamedrop and elemental blasts