Outside of the second town I keep encountering these Giant plant things that spew out a noxious gas and kill all my characters. Could some please tell me how to kill them first.

From far range,your mage must use to fire spell many times, so you can kill the plants.

I had only one Mantrap plant camping outside the gate after the sucker followed me. It still took me a few reloads to get it. I tried a few things but ended up making mself a little stronger on the other side of the town and then ran up to it and killed it. You can time your blows to some extent, when it rears back, quickly strike. If you have a bunch of them after your tail waiting for you, you may try running like hell past them. Use the shift and forward key to run. Keep going until you can heal up, but they do follow so run far (N and maybe E, to middle of lake?). They are a hard challenge.