The polearms are the weapons in the third row of the vanguard class. To get this achievement you will need to get 75 kills with polearms after you have unlocked the second polearm, so that is 100 kills total with polearms for this achievement.
These weapons are a cross between the vanguards two handed sword and spears. They don't do as much damage as the sword but they are quicker and they do more damage than a spear but aren't as quick.
Any suggestions for this solution just comment. :D