This achievement is best done in the low level servers due to the players being more easily killed there along with I found the less experienced players tend to choose the man at arms class more often than the experienced players. Other then that, just get 100 kills off opponents in that class.
Side note, men at arms will have a one handed weapon and will not be wearing much armour, generally found with a shield. They are basically the light class that is not a archer so they stand out.