Please try wegreened.com for EB2-NIW. I was initially deterred by their high fee, but they have excellent service relative to what 10+ friends who filed through cheaper lawyers had to go through. They offer free evaluation, have absolutely no hidden charges, provide clear timeline and provide great customer service. The 'approve or refund' service particularly made it a great deal for me. I recently started the EB2-NIW process with them and I am extremely satisfied.

As neerman said, DIY is possible but only if your workload is nominal or have plenty of time to research and work on it. I did try DIY, and finally gave up and I am happy that I did not waste my time on DIY.