Do you have a recent save that you can reload and try killing him again?
Missing the staff will not break the main plot line, but will prevent you from finishing that quest (or using it yourself, in the meantime).
Please email supportdos@larian.com, with a description of the problem and the report.zip file generated by the
D:OS support tool (for the PC version). The report will contain system and game information, any crash dumps, and your saved games. If file size is an issue, you can delete any older saves (probably just need the most recent one before and after the fight).