Can you get to download mode? This is accomplished by powering off device. Hold volume up then press and release power button while holding volume up. If you can get to download mode you could use the return to stock method described here:
http://xdaforums.com/showthread.php?t=2432476
I've soft bricked several times because of bad ROMs or trying to install TWRP. Once I had all the files, flashing back goes pretty easy.
I installed TWRP while being in JB 4.2, rooting with IORoot25 and the LGFree from the play store. It seemed like I had to install it twice from LGFree but that has been the least trouble for me.