So I've been playing for 4 years now and I am into death metal especially technical death metal. I can play stuff like Devildriver,As i lay dying, lamb of god(i know they are not tech. Death metal bands). But I want to play stuff like necrophagist,btbam,faceless etc. I want to rech technical level of the band i mentioned. Now I'm really confused what should I practice or learn. How can I build up such a technical level.
What about practicing technical death metal? If you want to learn to play technical death metal, practicing playing it will surely do the trick.

The key is just to slow down to a speed where you can actually play it, relaxed and accurate. If that means playing one note every 5th second, so be it. When i started learning to play progressive metal i had to slow down soooooo much to get all the wierd sections right, but then it was only a matter of getting more familiar with it and speeding up.

So that's my advice. Learn songs by The Faceless and Necrophagist and such. Slow them down until you can play them, divide the songs into multiple sections and then start putting them together.
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If you want to learn tech death, learn tech death songs, not other genres like metalcore -_-
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If you want to learn tech death, learn tech death songs, not other genres like metalcore -_-

That's exactly it, if you want to play like these bands, start playing their songs. In fact, the intro to Alaska functions as a fantastic sweep picking exercise, and the intro solo to Necrophagist's Stabwound can be made into many interesting little exercises that can improve your playing. Look outside of Technical Death, however, Power Metal is fantastic for the same techniques used in Tech Death, for instance, Speed of Light by Stratovarius, Wolf And Raven by Sonata Arctica and Unholy Warcry by Rhapsody. Prog Metal is also fantastic for this, any Symphony X or Dream Theater will improve your technique. I dare say, you've wasted a lot of time if you're still playing Lamb Of God after four years, so I'd say make up for that lost time by really challenging yourself, slow the pieces you want to play down, and speed them up little by little.