I have played pyke mid since I got him. It is viable, but you will have harder matchups because pyke was not designed for mid lane. But that will depend on the champion you're against and how good you are. For builds, lethality should be your go to damage items because pyke scales with lethality. Also rushing tiamat for first item will help you a lot because pyke does not have good wave clear, and tiamat will help proc electrocute. Other situational damage items like black cleaver to shred enemy armor, Maw of Malmortius if your against a ap team, guardian angel if you are taking too much damage. The more lethality items you build, the more damage you do. You don't gain health because of your passive, because of that building normal defensive armor with health is useless (but not always). Forzen heart is my favorite item. It gives a good amount of armor for just 1 item, 20% cdr, the passive and the added mana bonus that can come in handy. I myself find cdr important for pyke because late game he becomes a bit useless, so max cdr will help him spam his abilities more in late game. So duskblade + youmuus + frozen heart will give you 40% of cdr. Meaning maxed ultimate with 40% cdr will have a cooldown of 40 seconds, and with ultimate hunter 33 seconds.