So I ran one game as him in bots, first game, and went 4-0-30 (Surprise surprise, bots) well anyways, onto the build:
[B]SUPPORT
Masteries and Rune Page:[/B]
I ran him with AP focused masteries
Flat AP quints
Flat AP glyphs
Flat armor seals
Flat magic pen marks.
[B]Mechanics[/B]: His Q is a good distance way to mark them for your passive, so your ADC can get a few more hits and stun them for the kill. Whenever your ADC dives, if they're health is rapidly dropping, use your w to get in front, and e while backing up with them to escape. You ADC should really never die once you have your full utility kit. If your ADC is good, I'd focus maxing Q, and W, to dssh into the action, and be able to get that 70% slow for them to clean up the kill. If your ADC is bad, or having a rough start, depending on how good you are you can do the same, A and W, just for the get into the thick and get them weak. Should your ADC be really passive, I'd focus W, and E, so you can dash to them and defend them in a heartbeat.
[B]Build:[/B]
I rushed my [B]face of the mountain[/B] so I could get a slight sustain going, which would really shine in a PvP, with his toolkit of dash to an ally, and the shield on E, you should be able to front line with no worries.
My next was of course basic [B]boots of speed[/B] for the chasing, and running.
Next I started [B]Randuin's Omen[/B], for the active if needed, and brilliant tanking ability.
I started an [B]Iceborne Gauntlet[/B], just to do a bigger dent, and stack slowing abilities even more when attacking.
I'd recommend things like [B]frozen heart[/B] and such, to stack even more slow, and tanking abilities.
If you need magic resist, [B]Banshee's Veil[/B] is a cheap easy solution, if you want the health boost and life regen boost, you can always run a '[B]Spirit Visage[/B]' on him as well, as it provides all of the above.
[B]Video of Game 1:[/B]
http://youtu.be/u4Uh3ljkA_E
