The description of the achievement is not very accurate as it is impossible to stay under Rhodes' feed 100% of the time. But it required that you stay very close to his feet as much as possible.
When he jumps away from you, try to rush in again. When he crouch down to raise dead soldiers, stand at a reasonable distance and throw some spears at him. As soon as he stands back up, rush in. After all, fighting him from far away is not a good strategy anyway. So it makes sense that you try to stay close to him as much as you can.
The video is not mine and credit goes to the creator - Pascal Desrochers.