[X] Enter Glenhaven and gather information. If the Shrieking Talons have been here for a while, then the locals probably know some useful things that could help you out. This will almost certainly bring you into contact with members of the Talons, but that might just be more sources of information.
[X] Try to find Mantrae's people; they're probably nearby. As much as Robin doesn't like it, he agrees that it will be useful to have some backup. Besides, the Countess's agents will have been here longer; they might have a plan of their own that you can help with.
[X] Plan Extract and Fly - The goal isn't to kill the mercenaries, it's to get Sypha out. Besides, a frontal assault would reveal Ryza's existence and it's not like she would be willing to kill all the villagers to keep that hidden. Wait until night time. Ryza half-shifts, and Robin infiltrates the camp to find Sypha. When he does, or he gets caught, he uses a pre-arranged signal to call for Ryza who swoops in, grabs him and Sypha, and flies back to friendly territory.