This helped me out a lot when going for that sword.

First off there is a place where skeletons pop out on a bridge in barheim passage I think. Anyways that is a good place to get LP fast. The objective is to learn everyones quickenings. You can find where they are by scrolling between the members and seeing some of them have three misplace nodes that others have elsewhere. You can also look online to know what path to take. Once you learn all three quickenings for all characters you will have triple the MP you normally would. Also you will be able to use their most powerful attacks also.

So with that out of the way you just want to go in and perform quickenings with three characters until they are all done. You must chain them and try to pick the strongest ones each time. Then when all three characters are drained of MP, switch one of them out for someone who has MP. Use their quickenings and after you do this with all characters the demon wall should be dead or near death.

I did this myself. Sadly I went down the path and never got the sword. I had to wait until waaayy later in the game to steal it from a rare enemy.