You could conceivably do it if you have an Ulgu Black Magister and a window of time when no one with Windsight is observing the gyrocopter such as a time when for whatever reason both Mathilde and Adela leave the gyrocopter. Have your Black Magister sneak up to the gyrocopter, using Invisibility if needed in case there are any mundane witnesses, and use Substance of Shadow to phase a bomb on a timer into a sealed space, then drain away any remaining Ulgu to eliminate any traces of anything magical occurring and run away. When Mathilde comes back she doesn't see the bomb because by itself it isn't magical and the Ulgu from Substance of Shadow has been eliminated so there's nothing for her Windsight to see, the pre-flight checks won't pick it up because presumably they're operating under the assumption that sealed spaces with no way to access them won't suddenly contain anything unexpected, besides they're sealed so you wouldn't have any easy way of examining what's inside anyway. Then later when the gyrocopter is in mid-flight the timer goes off, the bomb detonates, the gyrocopter goes down, and Mathilde has to make a saving throw to try and survive the initial explosion and subsequent crash or die. At least that's how I would do it if I were the hypothetical Tzeentchian mastermind. It still requires an Ulgu Black Magister and a golden opportunity in the form of a time when both Mathilde and Adela are away from the gyrocopter and your agent is in position so a lot of luck is involved and luck isn't your friend when you're trying to assassinate one of the most devoted followers of the god of it but it is conceivably possible.
