#1
I tried learning this song but dude, it is really hard... tremolo picking for 8 minutes? In the beginning I play it clean but when I get tired my guitar playing gets sloppy.
any tips on how can I extend my hand's durability while tremolo picking?
#2
Practice. Practice. Practice. Oh, and I forgot to add, practice.

Tremelo picking for a whole song like that is kind of like running a marathon. You wouldn't expect to just sign up for a marathon and compete the next day having sat on your couch for the previous 5 years, would you? You have to work your way up to it.

A couple pointers though, to make sure you're not using more effort than necessary:

Pick almost entirely from your wrist, not your forearm. If your forearm is moving up and down, you're using waaaaay too much muscle to tremelo pick and you're probably also making your movements too large. Tremelo picking has to follow a very tight economy of motion in order to maintain accuracy and stamina. It even helps further if your wrist follows more of a "opening a doorknob" rotational motion as opposed to strictly moving side to side (as if you were shaking someone's hand by only using your wrist). That slight rotational motion is more natural for your body and more efficient.

It should not hurt, but until your muscles gain endurance, a slight burning will be present. Once you are certain that your technique is as efficient, controlled, and you're only moving the pick as far as absolutely necessary to alternate pick the string, all you have left is to put in your hundreds of hours to build up stamina. Then it's just practice, practice, practice.
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#4
i don't know the song, but i like to play a lot of Emperor which i imagine is similar. try to play the song really slowly (downstroke, quick pause, upstroke, quick pause, etc.). during the pauses, you should focus on being able to near-completely relax your entire hand and arm, maybe with the exception of the thumb and finger that is holding the pick. if you tense up your wrist excessively, your picking hand will have trouble moving at the proper speed. if you tense up your arm, you'll get tired eventually, even though your arm should have nothing to do with the picking.

sometimes you might be anchoring (either hand to bridge, or arm to guitar body) and will unknowingly start to anchor really hard as you get nervous while trying to keep up with the tempo. relax at all times! when playing fast, try to play the same way you would slowly.
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