Just attempted it today. I figured that attending the party in a unique disguise would be best, so I snagged the fireworks remote and knocked out Helmut. I got inside the party and followed him for a while, realising that he doesn’t really do much apart from wander about. I tested what the fireworks remote did and didn’t realise that I could perhaps sabotage the lights before the NPC’s were no longer distracted.
Plan B, knock out a guard, take the silenced Jaeger from the shed and climb up the piping to the second floor balcony. Then I subdued the waiter in that room and tried to line up a shot. Turns out there’s a massive tree blocking the Gazebo-thing. Waiting for his head to poke out from the gaps between branches, I can safely say it was the most tense I’ve ever felt sniping in a Hitman game. Eventually took the shot, struck him in the chest at first but then followed it up with a headshot. I escaped on the jetboat with no-one the wiser.
Then I realise I got spotted by a security camera right at the start and forgot to wipe the footage! Even so, with all other bodies hidden and never being spotted, only 1 star?? That doesn’t seem right.