This is a mission that sends you to make 15 double, triple, or multi kills using the Howitzer Gun. Meaning if you killed two enemies (bots) in a less than 2 seconds with the Howitzer Gun then it'll be counted as a double kill. Same goes with triple and multi kills. You must perform 15 double, triple or multi kills in order to complete this mission. There is no time limit specified for this mission so it is not necessary to do it in a single match.
"Make 15 double, triple or multi kills using the Howitzer Gun."