Does Shen's shield 2 ability let him become Mighty and trigger Fiora's legend?

He would become Mighty. Shen enters with his base Might of 3, gets assigned the defender designation, and only then does his shield 2 ability 'trigger' as a passive to raise him to 5 Might — that is a separate step from simply entering already at that Might value, so becoming Mighty at that point does trigger Fiora's legend. This is not the same as something like Trifarian War Camp's Might bonus.