I just did an FSC run earlier and took my Sentenza for some leveling. Since Vanaduke is allegedly neutral to shadow I figured it would be just as good as Argent Peacemaker for the boss fight. At first, it was. It performed the same as the AP in Phase 1 and 3.
However, in Phase 5, the gun suddenly did half damage. Confused by this, I later returned to Vanaduke, this time with an Umbra Driver.
Umbra Driver did 87 each shot to Phase 1 and 3. Still neutral. However, in Phase 5, no damage number appeared at all. Both the regular and charge attack failed to damage him at all. And yes, I made sure the shots weren't being stopped by flame orbs. I used the broken freezes to confirm I was actually hitting him.
So yeah, tl;dr Vanaduke is immune to Shadow in his last phase. Strikes me a very odd, so I sent an in-game bug report about it just in case it's unintentional.
uhhh it could be that you werent in range to see the numbers from the damage since u cant break freeze otherwise