I'm going to try to keep this short. I'm sure a lot of players know how to kill Roshan before leveling up to level two. However most of players don't even think it's possible, leave alone know exactly how to kill him.
So here is quick guide on how to do it.
Ursa and Skeleton King. It is best to do it with your mate or at least if you've made arrangements with someone who knows how to do it before the Hero Pick is over.
Note: it is best to select separately, not in the same time, or there is reasonable chance someone will understand what you're trying to do and that will be the end of it.
Vampiric Aura for Skeleton King and Fury Swipes for Ursa. I don't think I need to explain why.
Skeleton King: Ring of Basilius and a Healing Salve.
Ursa: Poor Man's Shield and a [Branch]].
Heroes attack Roshan, Skeleton King goes first, then immediately after goes Ursa.
The trick is in positioning. Roshan hit's the hero who's closer to him. He will beat you senseless so when Skeleton King has less than 20% he should walk off and let Ursa take the hits, immediately the Healing Salve on himself.
If you are confident in your ability to stay further than Ursa then go right back to the battle but it may be safer to wait until at least half of your hp is restored.
When Ursa has less than 20% hp you come in again giving Ursa time to vampirize some health from Rosh and increase his attack even further. Once you're below 20% again you just let Ursa do the talking. By now he should be able to vampirize more hp than Roshan damages him.
Be very careful with your hp. I can be so the moment you decide to walk off Rosh he'll stun you and you'll die. I don't think that was your intention when you decided to kill rosh lvl1.
Ursa must never stop hitting Roshan for more than seconds, or Fury Swipes will start again and you won't be able to output enough damage by the time Skeleton King has no hp left to spare.
Skeleton King must never go too far away from [[Ursa or he'll lose the effect of the Aura he needs to kill rosh.
Don't let other players 'help you' or be anywhere near you. It's likely they'll spoil the whole thing (unintentionally) and you'll end up reporting the **** out of them in frustration.
If you suspect the enemies found out of your sly plans - don't even think of going with it. Wait for level 6 and solo Roshan with Ursa.
P.S. Please tell me if I got any inaccuracies in the guide or if it's pointless. (i.e. common knowledge how to do it)
Otherwise share your opinions and vote it up! Thanks.