What I would suggest for a battle/diss beat is like a freestyle beat. Something very minimal, maybe only a drum break/drums, bass, and a very sublte sample/instrument. With those kinda tracks, you don't want peoples ears to pick up the beat as much as the lyrics, you want the lyrics to stand out, but at the same time have something fresh knockin in the background. Listen to "Top Billin" from Audio Two, you will notice that your not paying attention to the instrumental too much (besides the urge to head bang to the beat lol), but you will be listening a lot to the vocals a lot more than the beat since it's just drums (some nice ass drums at that).