Okay so im currently trying to learn Creeping Death by Metallica. I can play everything else rythem wise but I just cant get the speed right for the main riff. I can do it slow alright but when I try to build up the speed i just get confused lol and hit the wrong notes etc. Another thing that bothers me is that my wrist seems to tense up when trying to play it at speed, maybe thats the cause of the problem, i dunno lol Anyway id appreciate some advice from you guys
Keep practising, with a metronome if possible and gradually speed up. If you're making mistakes, slow down.
Be patient, it's a complicated riff and you won't do it right if you rush into it.
Yeah, patience is the key. Just practice it really slowly, until you know that you WONT make mistakes. It's different practising until you get it right, than practising so that you know you'll get it right every time. Slowly you'll start speeding it up.
Like the others said patience will prevail.

On a side note though, have you analaysed how your picking and fingering the lick, make sure everything isas efficient as possable.
If you feel your arm begin to tense up concentrate on relaxing your shoulder and the rest of the arm follows.
What the others said, but I thought I'd give you an example of how I apply this.
So, lets say I have a riff I want to play. Song speed is 200bpm. Sadly, I can only play it at 180 and even then it doesn't sound very good. So, here's what I'll do. Set the metronome to 100. Play the riff. If I can play it twice in a row, with confidence, and no mistakes at all, then I set the metronome to 110. Repeat process. At every metronome setting, I am not allowed to increase it until I have those 2 perfect reps in a row. When I get to 150, I'll start going up in steps of 5. Then lets say I get to 165 or 170. At this point, it's getting pretty difficult but I can still play it cleanly and it sounds good. Instead of the rule of the two perfect reps, I will keep going until I have 10 or 15 reps of it sounding good (not necesarilly consecutively). Then I will call it quits and move on to something else. Next day I hit it again, starting again at 100bpm.

After a few days something cool starts happening. I get to 165-170, where previously it was pretty difficult, and now it feels easy to play. Now I can increase the top speed to 180 again, except now it sounds good, though difficult. 15 reps. Move on. Before long, I'll have the thing nailed at song speed.

Of course, the numbers where just examples, but if you do this, when you do get to full song speed, you will own that riff.

Ok, suggestion #2 - you said that when you tackle the riff at speed, you get confused and start hitting the wrong notes. Where in the riff does this happen? If there is a pattern and you always mess up at the same point, you've just identified a weakness in your playing. Make up a lick or exercise (or play just that part of the riff) that focuses on that technique specifically, and keep working on that too. For example, if it's the quick hammer and pull on the 2nd and 3rd frets, then come up with something to work on your legato that uses the first and second fingers. It's really important when you have trouble with a lick or a riff that you're always analyzing and figuring out where the problem is and then practicing that technique.

Anyway, good luck!