Is there anyway to render passes other than combined separately ? I would like to render the Z pass, the mist or the normal pass separately but even if I disable the Combined pass, blender still renders the whole image everytime.
The Combined pass is useless in Cycles as far as I know, it doesn't change anything if it is enabled or not. In Blender Internal you can exclude from this pass other passes (like Shadow pass etc.) which might be useful, but not in Cycles.
If you are just after the normal, zdepth or mist passes, consider using some very fast rendering material on all objects (like material with no shader at all, just naked output node), because these passes depend only on the geometry and camera. Use the overwrite material option for Render Layers.
If you are using objects with displacement or normal textures, you will have to render these material for the normal pass to account for this. You can simplify these materials for bare minimum (a Glossy shader set to Sharp renders fast) and delete scene lighting to speed up the rendering. You can also limit the scene bounces or glossy/diffuse bounces, depending what shader you used.
If you are only after zdepth, you can also use just one render sample, the output is not nice anyway: Is Z-Render Pass accuracy dependent on sample count? Are others like object id? (Cycles) It's better to obtain this pass in a different way than directly.
If you need to customize how Blender saves and handles these passes, the place to do this is in Compositor. Enable it and connect the pass you want directly to the output node (and you can pretend the Combined pass does not exist). You can also use the File Output node to add how many file outputs you need to save all the different passes separately if not using OpenEXR Multilayer.