Can you break Grigori's horn?

[Eltia]

If so, do I just focus fire on its horn?

Also is it more efficient to farm Great Dragon's Horn from Ur Dragon?

[̷D̷a̷e̷r̷o̷n̷i̷c̷u̷s]

This is actually a good question. I have never tried this before.. +1 to your request.

[Spelunker]

The off-line Ur-Dragon is best for farming Great Dragon horns and claws.

[Karathrax]

I've never seen Grigori drop a Great Dragon Horn, in all the times I've fought him. He drops a unique material, Crimson Dragon Scale, though.

If the Great Dragon Horn is the material you need, Spelunker is correct that the more efficient way to get it would be fighting the offline Ur Dragon, or catch the online Ur Dragon when it's in Grace.

[szm]

ne dikka

[zadymek]

Yes, you can brak Grigori horn. It's not easy but possible.
Ur is a more efficient alternative.

[RobotParty]

Grigori does indeed drop a Great Dragon Horn but you really gotta either focus or get lucky. He's pretty tough in that regard, especially due to his size.