Worms W.M.D
29 Achievements
GOG
Grave Digger
Kill 500 worms.
1.4%
How to unlock the Grave Digger achievement in Worms W.M.D - Definitive Guide
Self explanatory; you just need to kill at least 500 worms across all game modes.
This should unlock naturally whilst fulfilling the other trophy requirements. If it does not, you can play a local game with 5 teams comprised of 8 worms each. Kill all of them to net 40 kills in a single round. Kills against your own worms do not count.