In particular, double kill and headshot occur very frequently, so they can get old fast. I think you have a line used for TERRORISTS that says "Extinguished" which could alternate with "Weakness" for headshots. Besides those two, there are a couple lines that are used for a bunch of moderately common achievements, such as "I am the vanguard of your destruction", "Rudimentary creatures of blood and flesh", and "Confidence born of ignorance". Achievements that aren't extremely rare that still need lines are the LAWNMOWER/HARVESTER and ACE/TOPGUN/SUPERMAN trees, QUAD DAMAGE/ROAD RAGE, PISTOL WHIPPED, and COMMISSIONER.

Ultimately I think it will come down to finding more sound samples to choose from if you want to flesh out the voice pack.