Hi,
I successful root my LG-H340n marshmallow a few month ago. It's a mix of several tips found on Internet.
Using USB download mode and "LG Root Script v1.3", it's possible to have an access to partitions and change recovery partition to TWRP. Using the same way I change also aboot partition (with one compatible with twrp).
After TWRP installed, I use it to install SuperSU systemless root.
And It's magic

now I can use my SD card and Apps2SD to create symbolinc link to my apps and free internal memory !
I have some notes and links, if you want, I can write a tutorial.