Dungeon Hunter: Alliance

It has been reported that this trophy might not unlock on a character with over 10,000 kills since the leaderboard for that character can stop updating. If this happens to you, you can simply grind out the kills with another class character instead for the trophy to rightfully unlock. Thanks to ESYLD for the confirmation.

This will come over the course of playing on Legend. There's no need to grind this trophy. Simply kill over 10,000 monsters with one character. You can also keep track of how many kills you have by looking at the Best Monster Killer on the Leaderboards.