The two handed axes are the weapons in the top row of the knight class. To get this achievement you will need to get 75 kills with a bastard sword after you have unlocked the second bastard sword, so in total that is 100 kills with bastard swords.
Free-for-All matches can be good for this as a swing from a bastard sword can kill more than one person with a swing, however these weapons are fairly good in all game modes.
Any suggestions for this solution just comment. :D