You're Comin' With Me - 10
Fragged someone after dying, before respawning.
Another tactic for this in any mode is to pick up the KAMIKAZE powerup and wait for a bot/friend to get close to you. Press your use key and you will die, creating a dome of death that will kill the bot/friend as well. The achievement will only unlock for the person holding the KAMIKAZE!
I can't remember which map(s) it's on, but it's the yellow SKULL icon when browsing the maplist.
Also, is THIS IS STROGGOS
online only? I've knocked bots off platforms for ages now and got no achievement!