Kill 30 enemies without dying.
Redeemer missile was shot down.
Killed the flag carrier within short distance of objective.
Hold 2 or more powerups at once (U Damage, Berserk, Invisibility)
Kill the person who last killed you within x seconds of respawning.
Telefrag an enemy (or telefrag a flag carrier?)
Kill a feigning player.
Complete a DM match without dying a single time.
Ignite someone else's Shock ball with your own beam, resulting in a frag.
Juggle someone with shock beam (hit them with second shot while they're in the air from the first shot) and kill them
One flag carrier kills another
Kill the same player x (10?) times in a single match
Kill falling player with direct rocket hit
Kill one or more players with an impressive moving combo
Kill a player with a direct full bio goo load
Kill a player and nail him to the wall with a stinger spike
Gib a player at close range with a full flak cannon hit
Gib a player with an impact hammer hit