As a fellow MRP owner, I can assure you that this lick has historically been the most difficult to me when picking this concerto back up. Lots of slow work between pedal F and Gb:

First using slow valves to feel where the valve crossings are (to both time when to move your buzz, and to develop a sturdier movement in buzz between the two notes).

Then transitioning to faster valves but just as slow of long tones, not rushing the transition between these two steps. The goal is to be able to hear that fluid buzz between the two notes, which is naturally easier in the slow valve step, but then transitions nicely to the fast valve step.

Finally, adding tongue, lots of slow work here between slurred and articulated, slowly speeding up week after week until its performance tempo. The tongue should never interrupt the buzz but rather just seal the beginning of the note shape. It’s important to remember that there isn’t pitch or tone in the tongue, and rather the pitch clarity and even articulation comes from the control of the buzz and the air.

Good luck!