Hi! Nice guide, I like the colors a lot. Here are my thoughts:
1. If you're going the poke runes, i suggest building Dark Seal + Refillable Potion as your start, since it synergizes so well with potions in general, and then you can turn in into Corrupting Potion.
2. Why not Doran's Ring start?
3. A good mana/CDR/burst option instead of Lich Bane could be Luden's Tempest. Lost Chapter is just a little bit more expensive than Sheen but gives 40 ap, more mana, same CDR and even more mana through it's passive, and Luden's Tempest gives even more stats.
4. In the matchups section, most are explained in detail, but in some, you should add some ways to counter play, like on Ahri for example.
5. In the overview section (and a couple more) you don't list Seismic Shard on your combos, is that on purpose? Why?
6. Maybe you could explain a little more your abilities and how to take benefit from them. For example, explaining that ANY damage, including minions, stop your shield from recharging, etc.
7. Any other summoner spell option? for example Exhaust against Syndra or Zed
8. In the runes section, you could explain all the three options you list earlier.
9. Maybe separate the offensive items from the defensive ones, and add a MR option like Banshee's Veil.

That would be it, overall I really liked your guide, and I know how powerful it can be, I've been playing a lot midlane these days, and I've seen 3 or 4 malphites.
Mini tip: Add Lissandra to your matchups, would like to see how you deal with her.