I tried a lot of cute strategies until I just tried standing in one place and brute forcing them down. You have to be able to use the melee attack (button 1), the ranged attack does very little damage.

I do something similar to what has be previously suggested, except that I just stand out in the middle and take my main back for a new mount (with slaves following - otherwise they will get charged for high dmg when you dismount). The reason I do it this way is it's much easier for me to deal with the "walkers" this way. I've got it down to where I can usually finish this off with only one mount change. Plus, it's very fast.