Dragon Age Nexus Mod
You will find four files to pick from:
-Will power ALL valid melee eliminates into deathblows.
-Tweaks the game a little so all Lieutenant level and above mobs (ones with yellow names) are required into deathblows.
Deathblow Lieutenants Plus
-Forces all Lieutenant degree mobs (and above) into deathblows, plus it ups the likelyhood of deathblows happening with regular mobs besides. I would suggest using this one.
-Does the alternative, and prevents deathblows from occurring to anybody EXCEPT legitimate boss mobs (ogres, dragons, etc)
Install is not difficult, only UNZIP and toss ONE of the .gda files into your override folder that should maintain:
C:\Program Files\Dragon Age\Packages\core\override\
depending on for which you installed dragon age to.