The first thing you should build is Nashor's Tooth or Wit's End. Both are situational. Wit's End in case you are against an AP champion or a tank, like Nautilus or Zac. Reminder that buildling Wit's End will take away the CD benefit on Nashor's Tooth so your "Into the Fray" E won't come up as often. Take a Morellicon somewhere in the build later IF you took Wit's End. Though it's not mandatory.
Next is Runaan's Hurricane, followed up by lifesteal item. Hextech Gunblade or Blade of the Ruined King.... That's your choice.
Now, those are four item slots done, you build just a bit tanky. Guardian's Angel and Frozen Mallet.
Path, max E first for maximum splash damage. Even while they farm, they'll get harassed.
Try not to get close.
Your Q is the second.
When you are being ganked, Q whoever has the better gapcloser, and W yourself and run. MS boost will be more than enough. If it doesn't, burn flash or ult.
Jax is a problem. Try to QW away when he uses counterstrike.