AP Malphite's core item is Zhonya's Hourglass.
Expose Weakness , Swiftness and Oppression are pretty horrible masteries, and Malphite doesn't really profit a lot from Second Wind, so you might want to switch those mastery points a little bit in something else, like Resistance in the defensive tree and Butcher in the offensive tree. As mentioned before, Greater Seal of Armor is the best way to go with Malphite, as it also gives you a little boost to your Ground Slam while it also reduces damage from autoattacks and minions in your lane.
Other than that, I think this is a decent TL;DR guide on how to play Malphite mid, and I'll upvote if you fix those little errors mentioned.

well...excuse me...i will give you a second chance if you like. Add the possible team comps and explain why ap maplh can be strong in these situations. If you do so, i'll upvote. Fair enough?

Not saying AP Malph is viable, but it kinda is. The only problem is that it is ureliable. However, it could non-doubtably work in solo que. However it would require Zhonya's Hourglass as a core.

Basically how it would work is you try to land an ult on as many people as possible, which isn't easy, but possible. If you land a good Unstoppable Force, which has 100% AP stacking. The enemies would probably be around half health, along with Ground Slam. And by the time they land from being knocked air-borne, you would have Zhonya's Hourglass active to save you from being deleted. By then the team fight is basically a clean up for your team.

If you took the time to read my guide the very first sentence says this: "It is not a very popular pick but I still think it is strong and very useful depending on team comps". I am very aware that AP Malp Mid isn't going to work always and in every team comp. It just happens to be quite useful if the right conditions are met and I haven't seen a proper AP Malp Mid guide yet so I decided to make one. Thanks for downvoting without reading though.

if you're refering to alex ich or any other pro i can tell you the following...he made it work because he was in the right team composition and under the ideal circumstances. None team in solo queue will is so well-coordinated and AP malph will just be rocked

Thank you so much for your useful input. Have to say Alex disagrees.

Thank you for your feedback, really appreciate it. Guide was created when I was bored so it lacks a bit in detail and BBcode, depending on reactions I'll update it and make it more extensive.

The weakness of this build is that it has little armour in its items thus makes it more vulnerable to it so what works for me is using Hardiness , besides there are two items which give Magic Resist which I feel is enough. The skilling is corrected now, my bad! Taken into account your advice on seals.

i'll just downvote cuz AP malphite mid(or anywhere) is way too sub-optimal not to say NOT VIABLE.

A few things:

• Greater Seal of Mana Regeneration is a big no-no. It is more than enough you have a Chalice of Harmony and a Doran's Ring.
• Your skilling order is impossible to be true.
• Two points in Hardiness ? What about Resistance ? You are going mid lane after all.
• Guide lacks BBcode.
• Guide lacks detail.

Thats all for me. Best of luck with the guide!