First of all i have to say that this guide is just for having fun.
But still, never underestimate Alistair mid.
My English is not perfect but i hope everbody can understand what i want so say :)

Pros:
+ Dominating Laning Phase
+ Good Teamfight initiation
+ Earlý level damage
+ A LOT of Fun


Cons:
- Does not Scale good into Lategame
- Near Useless if you loose your lane

Laning Phase:
Basicly thats the strenght of AP Alistair,
Take your w first and deal some good damage by kicking the enemie away from the minions.
When hitting level 2 the "Nuklear Slap"- Thing beginns.
The "Nuklear Slap" is just ur W+Q combo, get one on the enemie and then backoff while they are cc'ed.
The Second W+Q+Ignite+Passive+Basics should be enough for a kill. If you dont think its enough better backoff without Ignite and basics. You should force the enemie out of lane anyway.

Do that again until you are confident to Roam.
Start Roaming and get some kills :D
Btw --> Alistair goooooooood KS Champ ^^

In Teamfights try to focus an easy to burst target and get them out of fight really quick, wait for Cooldowns and go for the next target :)

If you play him some more you will undersntand why its "Nuklear Slap", trust me!

Dont forget:
Alistair Mid is a Fun Build, i do not recommend playing it in Rankeds and dont take it so Serious.

And as always:

Have FUN!