As someone that plays alot of metal it just takes time to build the endurance in your arm. You wouldn't be able to run a marathon your first day running. Make sure you are using proper technique though and not one that will damage any nerves in your arm.
Just teaching myself. I don't know how it could be improved - but Rocksmith is lacking primarily in the way of feedback. You can easily hit the correct notes but do so in a way that forms terrible habits. On the flipside you can miss sections and have no idea how to correctly play it. There's also the lack of music theory but many people, myself included, don't care too much about that. Lack of direct feedback from someone with proper training is probably the biggest issue for me. Perhaps the next RS could feature some sort of in-game lesson system via skype. I've also noticed that RS can act as a crutch in learning songs - you really need to push yourself to play without the visual notes to learn a song.
Go to Track -> Pro Guitar -> Set Tuning String 1 is your skinniest string and 6 is the thickest, so from 1 to 6 your settings would be 0, 1, 0, -2, -2, -4
I usually just chart it as a bunch of notes in ascending/descending order. There's no notation for it in RS14 but once they player knows the song he'll recognize the pick slide