I cannot set up the cannon at the spot you are supposed to for the 8th Bartalli's adventure log, it keeps telling me "go to the marked location" but the marked location is on a ledge on the side of the cliff, not on the castle wall that every guide I have seen has pointed to, and I can't complete the logs because of this. I spoke to pearl abyss but all they said was that they know and they are working to fix it, which that conversation happened months ago and it's still not fixed. Anyone got any suggestions for a way around this?

I am trying to do the same thing, and I am not even getting a location mark. :(

Just jump to the ledge before the bridge to the artifact. You fire from there.

Having recently completed this I can confirm that the ledge on the side of the castle wall is the correct current location to setup.  Some older guides have different locations and these are now incorrect.

So, rent the cannon and cannonballs, make your way to the ledge, drop down, setup the cannon, fire the 10 balls (or take 10 minutes) then the cannon falls apart.   now you need to get back to the blacksmith and start again?  You did use a lahn didnt you?  FYI I hit the area on the 4th round of 10 canon balls. Just a bit frustrating.