I have used full strength to do the job of killing an established clump of bamboo that would have been impossible to dig out, or at the very least back breaking!
Just keep in mind if you are not diluting it to manufacturers instructions be more careful with spillage and not to let any people, pets/animals near it while it is still exposed, and to wear protective gloves etc.
I have also found that it is best to do this in damp weather believe it or not. The plants seem to absorb it to the roots more readily when they are taking in moisture than they do when the weather is dry. Having said that, it could be my imagination!
Although I have used undiluted round-up myself, this is my personal opinion and should not be taken as expert advice or in preference to any instructions on any product you use. Using round-up undiluted will be your decision/responsiblity.
My disclaimer is linked from the footer of this page.