Item's and General info about them

OK so you're probably wondering why I haven't picked Rylai's Crystal Scepter or Liandry's Torment. You're thinking that because of Teemo's Toxic Shot passive on each hit doing extra AP damage. Well its not considered spell damage therefore, doesn't activate Rylai's slow passive or Liandry's magic burn passive. Just wanted to get that out of the way.

Ok so you see the base build. I highly recommend rushing Nashor's Tooth as it gives good AS and AP as well as the fantasic passive of doing 15 + 15%AP as bonus damage on hit. Since that will increase Teemo's base AD as well as AP and AS, its a fantastic first choice for major item.

After this, its whatever order you feel will help you the most. I generally recommend Rabadon's Deathcap as a second major item as it will only lead to further your AP which in turn, because of Nashor's Tooth's passive, increase your AD even more. Whenever you go back, try to purchase an item off the list, as you can't just go to base and purchase whenever you want because ARAM.

If you find that their tank is taking too long to take down, get Blade of the Ruined King. If not just continue through the list. Make sure you are getting a good amount of kills, if you aren't because they keep getting away, get Phantom Dancer sooner. Also note that you will sell your boots to get Phantom Dancer. Sell Ancient Coin last, upgrade it one level if you find you're dieing alot. Teemo is a very item dependant champ mid-game.

Situational Items

I'll keep it short and simple.
Get Void Staff instead of Deathfire Grasp if more then 2 enemy's are building MR. If the 2 that are building it seem to be your main target's, try to change targets, if you can't, then switch out for the Void Staff

Get Abyssal Mask instead of Hextech Gunblade if the enemy team's AP cannons/poke's keep killing or majorly damaging you. Remember to move around alot as you're Teemo, he's very squishy.

Get Zhonya's Hourglass instead of Hextech Gunblade if the enemy's AD carry keep's focusing you in a fight. When he comes at you, right after he activates his first ability, use stasis, he'll forget about you and move on. He will take some damage, maybe come back to you after you're out of stasis, at which point you can mow him down and take a little of his damage.

Get Infinity Edge instead of Abyssal Mask if 2 of the enemy are building alot of MR. If 3 or more are building tons of MR, the do the above, and also switch out Rabadon's Deathcap for Sanguine Blade as well as Nashor's Tooth for either Statikk Shiv or Zephyr.

Strategies

Hug your turret and move around alot. ALWAYS enter team fights last. Use Blinding Dart on the AD carry when he gets into the fight and shoot down either their AP Carry, AD Carry or Tank (in that order). FOCUS on one enemy at a time, do not try to shoot everything.
Put your shrooms into the bushes, if you find your team keeps getting pushed back to your turret, flood the very edge of the bush closest to your turret with shrooms. If the enemy team keeps bunching up during team fights, put a shroom or two in the center of the fight as it will damage everything then get outta there. Remember your squishy.

Thats really it. Have fun Teemoing around.

WHY!?!

You may be thinking, WHY? WHAT ARE YOU ON? Well I recently found out AP stacks fairly easily, on top of that, with the help of Nashor's Tooth your AD will stack with it. Since ARAM is mostly a poking contest along with AeO spells, Aatrox becomes fairly useless due to lack of ranged attack. So basically I devised a way for him to be devastating in poking and AoE while still being a decent AD carry.

Items

I have no real build order for Aatrox except for build Nashor's Tooth as fast as possible after the starting items. Liandry's Torment is a great second major item as it is totally OP with Aatrox's poke, Blades of Torment. Liandry's does double damage to movement impared champions, so basically you are doing 12% of their current health in damage, besides the base damage, every time you hit someone with Blades of Torment since it slows. After these two, just build in whatever order you want.

How to use

Ok so since you are now not really playing AD carry, Dark Flight should be saved for either retreat or chase, not to engage. Leave your W skill on Blood Thirst until you get Rylai's Crystal Scepter. During big team fights, stay a few steps away from your tank, as soon as the enemy team rushs him, use Massacre and laugh at how all of them are at 1/2 to 3/4 health (subject to how much MR they have). After that shoot off your Blades of Torment and then GTFO. If someone starts chasing you, wait until Blades of Torment are half-way done cooldown, then go and start attacking him, as soon as you shoot of Blades of Torment at him again, he will either die or be very close to death.