Chasing the enemies in Betrayer is quite boring. Why they just don't appear in a bunch of 10? Or wait... They do! After returning to Fort Hope from Forbidden Wilds and speaking to Allison there is an attack of conquistadors. Kill them, then press Alt+Tab, stop the game process in Task Manager (to avoid saving the game), then start it again and continue. You'll respawn right at the beginning of the attack. Repeat as many times as you need to earn the achievements. Every time you'll add about 10 kills to your count.
If you have completed the game and you can't return yo the moment of the attack, try other save files. They are here: c:\users\<username>\Documents\My games\Betrayer\Binaries\Win32. There are 4 files: game.sav (the last save of the game) and the files named like archiverecent-X.sav. Just rename game.save to current.sav to keep your game progress, then copy any of the archive saves to game.sav. Maybe you'll find the right moment this way.
Good luck!