What I have figured out on Rosh.

Okay so obviously the new meta is going to require Omni as his repel causes Orchid to deal the damage right away. I've been trying to figure out exactly when Omni should hit repel to kill Rosh, negating the issue of waiting for Orchid to deal damage. This is what I have so far:

• Orchid: 30% bonus damage (All dealt when repel is put on Rosh.)
  
• Pugna Decrepify: 40% damage (Echo Slam damage is negated, but I don't think this is an issue.)
  
• Veil: 25% damage
  
• Undying Ult: 35% in close proximity/20% from further away (Boosted by scepter)
  
• Etheral Blade: 40% bonus damage

So I put 'D' as the minimum amount of magic damage (without amp bonus damage)

(D + .40D + .25D + .35D + .40D) x 1.30 = 150,000

2.4D x 1.30 = 150,000

2.4D = 115,384.6154

D = 48,076.92308


So when Roshan hits 101,923 HP, Omni should cast repel (Not sure on the cast time of Repel, but it seems pretty long) on Rosh to kill him instantly.

My math could be off, so if anyone has any corrections or thoughts, post a comment below.