It will be fairly common to lose your sword while fighting, when scrambling around. Just pick up your sword again by picking it up with

. You will likely need to roll back over to it (

+

or

+

).
This is not technically unmissable, but you would have to be trying very hard to never drop your sword and always die before picking it up.
For all achievements in the game, you can follow this awesome guide on YouTube:
All credit to Trophy Tom. Please like or subscribe to support him. Please note how he has conveniently labeled sections of his video based on which achievement is next or what item you are tracking down, this can be seen if you watch his video on YouTube on the video progress bar instead of the imbedded video here on True Achievements. It is a very well done 100% guide.
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