I actually miscalculated some magic penetration, so I do slightly more damage than indicated.

Assuming I'm Aatrox with Sorcerer shoes, Liandry's Torment, Ravenous Hydra, Blade of the Ruined King, Guinsoo's Rageblade, and Hextech Gunblade.

First, calculate his AD.
At level 18, Aatrox will have 115 base AD.
Runes will give me 6.75 additional AD.
Masteries will give me 14 AD.
Blade of the ruined king gives me 25 AD.
Ravenous hydra gives me 75 AD.
Guinsoo's will give me 30 AD.
Hextech will give me 40 AD.
This totals to approximately 306 AD.

Aatrox has 201 bonus AD.

However, warlord will give me an additional 15 AD, totaling to 321 AD.
Next, we calculate his AP.
Liandry's gives 80 AP.
Hextech gives 80 AP.
Fully stacked rageblade will give him 72 AP.
This totals to 232 AP.

Let's say I'm bursting a Tristana, who is building purely damage.

Aatrox Q, after calculations, will deal 371 physical damage.
Aatrox's W, in blood price (Damage form.) , after calculations, will deal 401 physical damage. In addition to his AD, this will deal 707 physical damage.
However, this auto attack will deal an additional 155 physical damage due to BotRK.
Aatrox's E, after calculations, will deal 475 magical damage.
Aatrox's R, after calculations, will deal 632 magical damage.
Throughout the duration of this burst, liandry's torment will deal 232 additional damage assuming the target is only burned for 3 seconds.
This burn damage could be higher but for simplicity's sake we will keep it at 3 seconds.

In total, this burst is 2572.

However, with the 5% additional damage I will receive from masteries, the full rotation of his burst will deal 2701 total damage.
This damage is composed of 1265 physical damage and 1436 magical damage, basically split even.

This is from purely Q, W Blood Price, E, R, BotRK Passive Damage, and Liandry's Passive Damage.

Taking into account her armor and and my armor penetration , she will resist about 38% of my physical damage, making my total physical damage be 784 damage.
Taking into account her magical resistance and my magical penetration, she will resist about 19% of my magical damage, making my total magical damage be 1163.

The total burst will thus be 1947 damage, meaning Aatrox can essentially one shot Tristana, who has 1937 health at level 18.

40% of this is physical damage and 60% of it is magical damage.

If we add the actives from Items, Hextech deals 243 magic damage, Ravenous Hydra active will deal 306 physical damage, and BotRK active will deal 194 physical damage.

Taking into account Tristana's resistances, Ravenous Hydra and the BotRK active will deal 326 additional physical damage, and hextech gunblade will deal 207 additional magical damage.

If we add the active damage to the normal burst damage, we can safely conclude that Aatrox's entire burst will deal 2480 damage, 1110 being physical damage and 1370 magical damage.

This shows that while Aatrox can kill Tristana without the actives, he can further deal even more instant burst than initially expected.

This is not taking into account the insane lifesteal, attack speed, and spell vamp that Aatrox can use to further heal/DPS other targets to death.

TL;DR: My Aatrox build easy one shots ADCs with plenty of excess damage available.