I've also got slaven and I tend not to use the big amorphous blob, but more use a big central brigade including the streaming Bell supported by a nummer of satellite brigades.
Some of these I use to soften up my point of attack before I engage with my main force. Others are used as a reserve in case things go wrong. I find this way of playing much more versatile than the big blob.
To protect your army I find the gutter runners Invaluable. Eventhough I usually Will have Some in the army, I keep some back to pop up in terrain to make it more difficult for cavalry or other fast units to pass. Either the opponent Will try to pass, but risks failing a command, usually exposing a flank, or the flank attack gets cancelled as he withdraws his fast strike force.
To soften up a target the Death Frenzy spell is quite a nasty surprise. In my last game with slaven versus Jo's witch hufters, I cast it on a rat swarm assaulting defended crossbowmen supported by halberdiers and came out on top.