Hey, all...
Using Shabbypenguin method, I was able to root not one but two Galaxy Light t399n (the Metro pcs version) phones yesterday. Super easy! My advice to anyone else wanting to do this but unsure of how to do so would be to get the needed files from shabbypenguin.com and follow the instructions on the following website:
http://theunlockr.com/2014/08/17/how-to-root-the-samsung-galaxy-light-t-mobile/
This set of instructions are the most clear, detailed set I have seen anywhere on the web. If you can follow simple step-by-step instructions, you will have your phone rooted in no time. I rooted two t399n phones yesterday... used the instructions on the first phone (my phone), didn't even need the instructions to root the second one (my gf's phone). It is literally that simple!
Do not download the files they tell you to download on theunlockr... you must use the file versions from shabbypenguin.com to root your t399n, as the files suggested by theunlockr are for the t399, which is the T-Mobile version of the Galaxy Light, and will probably brick your phone to some extent if you use them on your Metro pcs phone. Also, two steps generally not included are: 1) you need to download the usb drivers for your computer to recognize your phone in Odin, which are on the Samsung site, and 2) you have to set your phone to usb debugging mode. To do this, go to Settings > More > About device... once in the About device menu scroll all the way to the bottom until you see your Build number... tap your Build number 7 times, and developer options is unlocked (you will receive notification of this), which can be accessed in the More menu by simply pressing the back button on your phone. Go into Developer options, and enable usb debugging. These two steps I just told you about MUST be done before following the instructions I posted from theunlockr.
I have noticed while reading a few posts that when flashing TWRP to your phone using Odin (per the instructions), some people reset their phone when the green indicator in Odin says 'reset'... don't do this. Patiently wait until the green indicator on Odin says 'passed' before moving on to the next instruction (removal of the battery).
Found all this out by perusing the net for a few hours, took the plunge, and now I'm rooted (x's 2). Glad I did it. Thank you shabbypenguin for posting those files. I'm a noob to all this phone stuff (though I do have a pc tech background), btw.
Sorry for the long post... saw people struggling, wanted to help.
Cheers!