The chageling is also a threat you cannot realistically prepare for, so there's little point worrying about it.
On more serious plans, the gyrocopter is our weak link there, as our routes are somewhat predictable, and we'd be potentially alone, or with a dozen others at most. It I were a tzeechian sorceror, I'd send some agents to figure out when mathilde is gone from K8P, and ask when she's coming back. That gives a rough timeline of when a gyro-transport is coming in. Trace the likely route and set up just outside the range that the undumgi can plausibly respond to in a reasonable amount of time. When Mathilde shows up, use some spell or other to take down the gyrocopter, and then have my cultists/deamons/mercenaries/whoever attack whoever survives the crash, while I rain down magic from farther away. Depending on how "just as planned" I can get, maybe arrange for a largish orc/goblin war party to be in the area, to form the first wave of the fighting.
Now this is hardly a foolproof plan given that wizards make up a good chunk of the gyro's passengers, and a good chunk could plausibly survive a crash. That leaves a lot of magic on the table - including battle magic - and getting into a counter-spell duel with Mathilde has ended poorly for enemies before, but something like this seems the most plausible assassination method I can think of.
