You can totally learn metal (its not that hard for an intermediate player).

Speed Exercise:
1. Choose a lick or a run that you like or are having a hard time with
2. Start the metronome at a slow sped for you where you can play it perfectly
3. Mark that speed on a chart
4. Increase the speed by 5-10bpm and see if you can play it perfectly at that speed
5. Rince and repeat until you get to a speed you cannot play it perfectly
6. Mark that speed on a chart (max)
7. Back off by 5-10 bpm and practice there playing it perfectly 10,20,100 times in a row.
8. Do this every day and watch your speed increase.

How to increase strength even when you are not playing:
1. When your hand is not doing anything, get in the habit of tapping different alternating fingers in different patterns.
2. Get a gripmaster. These are spring loaded devices that you can squeeze that will get the fingers stronger.
3. As crazy as it sounds, I’ve found doing pushups and consequently strengthening the forearms, can make a huge difference to how easy it is to grip the guitar and press down on the strings. It all has to do with how the muscles are attached and different points.

I know a website that has more details and learning techniques. The link is below.

Good Luck Shredding,
John
References :
Great tips for learning guitar:
http://www.masterysites.com/guitar-training/

Put all 4 fingers on consecutive frets on the 3rd string (ie. 1st finger on 1st fret, 2nd finger on 2nd fret, 3rd finger on 3rd fret, and pinky on 4th fret). Practice just pressing and releasing with all 4 fingers together but don’t lift any of the fingers off the string when releasing. Now do the same with one finger at a time, pressing and releasing, but leaving the other 3 fingers touching the string at all times. This gets your hand used to the amount of pressure to press and release the string. You don’t want to press too hard or release too much that your fingers are flying up off the strings.
Now the tough part, with all 4 fingers pressing down the 3rd string, release the 1st finger and move it up to the same fret on the low E string, then release and drop it down to the high E string. Then return it to the 3rd string. Seems easy so far, but wait! Now do this with the 2nd finger. Still not too bad. Now try it with the 3rd finger. Your pinky will want to move too and it might actually hurt to try this. Don’t pull a muscle, but keep practicing this motion. Do the same with the pinky.
If you do those types of exercises regularly, your fingers will be flying pretty soon!

You can get one of those spring finger exercisers or a squeeze ball thing, or just walk around wiggling your fingers in the air when you don’t have a guitar, but the best thing to build strenght is to play often and do these types of exercises.

Ripping through pentatonic scales is kinda fun too!
References :