Park Tool DW-2 Shimano Rear Derailleur Clutch Wrench

It might be small and thin but it's the perfect tool for the job.

The Park Tool DW-2 Shimano Rear Derailleur Clutch Wrench has both 5.5mm and 3mm openings and is designed to fit the rear derailleur clutch tension adjuster on Shimano XT, SLX, Deore, Saint and Zee rear Shadow Plus (+) derailleurs and all previous Shadow Plus models.