The spears are the weapons in the second row of the vanguard class. To get this achievement you will need to get 75 kills with spears after you have unlocked the second spear, so that is 100 kills total with spears for this achievement.
The spears are fairly fast for a two handed weapon so be sure to try and use that to your advantage. Of course they are not as fast as a man at arms with a one handed sharp, but they can beat knights and vanguards with other types of two handed weapons.
Any suggestions for this solution just comment. :D