With a +10 weapon this should absolutely not be the case. You must be sinking points into a stat that your weapon does not scale with. There is no reason why a +10 weapon and a level 30+ character should not be able to deal decent damage to the magma wyrm.
My last playthrough was starting with a sam, grinding until level 40, then going to get Moonveil. So something is wrong with your setup.
Honestly I don't remember if I tried with +10, I definitely did with a +6...
This is where the game's bigness gets to me. So many things to keep track of.
I'm gonna keep going with my current set-up until I get bored or annoyed, I'm enjoying it. This morning while absent-mindedly running around the game while doing other stuff I stumbled into the Queen boss in the Academy. I was doing real good damage, I only lost because I didn't realize there was a second phase and put my controller down. So in terms of actually progressing through the game and holding my own in important fights, I'm good for now. I really like the Grossmesser.
Keeping in the spirit of "you can go back to things later" that is Elden Ring's selling point of the critics, I'm fine with waiting on the Moonlight Katana then.
I know I'm complaining a lot but that is kind of the appeal of these games- that I think about them and care so much about a state and a weapon and a boss.
Yesterday I had a fun moment- I was seeing footage and artwork of those enemies that are like robots on wheels with massive blades whirling about and it also grabs you and eats you. I figured oh man the first time I see one of those it'll kill me. But I encountered my first at the academy and took it out with patience and skill and my upgraded weapon. Felt good. A couple crabs crept up and I took 'em out, and I was feeling from them earlier in the game.
The Academy with its blue color scheme and library vibes reminds me of that Dark Souls area that ends with the dragon fight, I like that. I will fondly remember this nice little area when later on I'm dealing with poison and snow and all that other bullshit I know is coming.