There's two approaches, to me:
- Get 5 high damage heroes on the same team, put utility heroes and tanks on the other team. This is more resistant to error since you have backup in each role in case someone messes up and dies. This is how the previous top score was set by Invictus Gaming: http://i.imgur.com/6gnRtJR.jpg. It looks like they used the Pudge with Flesh Heap buffs almost exclusively, simply buffing him. This no longer works, however. The next approach used Heaven's Halbred to disarm Roshan, which also no longer works.
However, I'm pretty sure you can do better:
- Put a tank to be cleaved on each team. This is not very resistant to error because if one of the tanks dies, the damage output of your team drops by about half. However, you get more attackers so you'll be more able to deal more damage later into the game. This is the better approach, assuming no one makes errors (and since you have unlimited tries, this is probably how the best scores are going to come eventually).
I'll describe the second approach:
Cleave deals a percentage of your damage based to all units in the area. All different cleaves stack additively. I am not sure what the ideal build is for the DPS heroes, but it's a combination of Battlefury, Daedalus, and Divine Rapier. My thought is probably 4 Battlefuries, 1 Daedalus, and 1 DR in the general case, but it's definitely different for different heroes. You should also have 3 Refresher and 3 Boots of Travel in your stash, and scatter plenty of free Divine Rapiers near the Roshan area for when people die and need to reclaim them. You have time for this early on when the kills are slower.
Utility heroes should be carrying Hex, Orchid, and the necessary armor debuffs to apply to the enemy tanks that are being cleaved: Deso, Medallion, and AC. The spacing for AC will be tough so that you can debuff the enemy tank but not buff your tank. Lion and Rhasta seem your best bets, as hexing Roshan with a Sheepstick causes you to also be hexed.
- DPS: The likely candidates: Phantom Assassin, Alchemist, Troll, Lone Druid, Bloodseeker, Magnus, Sven. Three per team.
- Tanks: Dirge Tiny, Abaddon? Buy HP if it helps, and whatever else keeps you from dying to Rosh. Blink, Force Staff, Euls? Dunno. One per team.
- Utility heroes: Lion, Shadow Shaman. One per team.
Other stuff to consider or that I don't know that might also help:
- Doom? Wolf aura is pretty good, and does the spell itself do anything to Rosh?
- Any way to have a dominated wolf creep stick around?
- Can you purge Roshan's physical immunity with Diffusal Blade?
- Ethblade+Veil+Dagon nuke combos, then purge the Ethblade?
- When do your abilities get refreshed? Optimize for best buffing?
From then on it's really just a ton of refinement and finding out which combinations of heroes on each team help each other the best. Maybe it IS actually better to stack 5 damage dealers on the same team so they can buff each other more. It's possible but unlikely that just going all out and buffing one guy (Pudge with Flesh Heap) is the best. Maybe there are ways you can get around having disablers, such as with mass Basher.
But yeah, it's a big time investment to actually test all of this stuff out and figure out the best approach. Good luck!