I would give a warp lock engineer the brass orb. A very powerful wepon. Considering the model is only 15 points, it's low risk with high reward. Put it with a group of clan rats and move them up, detach it from the unit and go for it. Also I wouldn't take slaves unless I was taking a warp lightning cannon or a doomwheel. Considering their expendable special rule you can shoot into the combat they are in! Also they can be buffers for the doom wheels. Taking the hits for the rest of your army.