Tony wants you to steal the diamonds back from a meeting at the museum. You're going to need Yusuf's helicopter for this though.
The Impossible Trinity
Make your way slightly West to the helipad and pick up Yusuf's helicopter, then fly it to the rooftop of the museum in Middle Park. Drop down the scaffolding and take your position at the window to watch a cutscene you'll have seen two times already if you've played most of GTA4 and The Lost And Damned.
After the cutscene is over where you've gotten the diamonds back, climb the ladder to the roof and get in the helicopter. Fly away from the museum and Luis will call Tony. A police helicopter will come after you, so you need to shoot it down. Use rockets (my preferred method) or the gattling gun and take it down. Two more helicopters will then come after you, so use the same tactics to take them out.
Once they're all destroyed, fly the helicopter back to the pad where you got it from and give Tony the diamonds.
Mission 100% Completion Criteria
In order to get 100% completion for this mission you need to fulfil the following criteria:
- Time: 04:35
- Player Damage: 20%
- Helicopter Damage: 80%
Mission 100% Completion Tips
The only thing you'll really need the time for on this mission is taking down the three police helicopters. If you can do that quickly, and you do everything else as quickly as you can, you should be under the time limit. You shouldn't take any player damage, and unless you get hit by multiple rockets, or fly the helicopter into buildings you should pass the heli damage objective fairly easily too.