Here are some ideas:
STALKING PREY: Kill at least 6 enemies in one match while under the effect of an invisibility potion.
BARBEQUE: Kill 4 enemies in one team fight match without dying using the flaming volcano.
PROTO-TRIPLE: Kill 3 enemies in a row with one round from any version of the Prototype.
GET MOWED DOWN: Kill 4 enemies in rapid succession with any version of the Automatic Peacemaker.
SAMURAI: Get 5 kills with the katana without dying.
SUPER SAMURAI: Get 5 kills with the katana while wearing the Great Samurai Helmet without dying.
DATER DESTROYER: Kill 15 enemies in either Paradise Resort or Heaven Garden without dying.
VINTAGE KILLER: Get 20 kills in one match in D-Day using either the State Defender or Brave Patriot.
I still have tons more of ideas, but I don't want this post to be too long, so yeah.