I copy the object used for cutting, paste it in front of each object and use minus front repeatedly for all objects which is not a big fun for a larger group of objects. Is there a faster way?
I don't want to use a clipping or opacity mask because I will be deforming the object. Actually I don't want to remove the upper part if this is possible but to separate the upper and bottom parts as two independent groups.
If you canvoert your strokes to paths, you can use a pathfinder operation. You need both parts, so first create two clipping masks (upper and lower part), then use the pathfinder "Crop" on them to cut permanently
Draw a straight path where you want the cut to be.
Keep it selected, then Object > Path > Divide Objects Below.
This works pretty well on most things but you will need to make some special adjustments if there are filled and stroked objects that need to be divided.
Otherwise you will get strokes along the dividing line.
You are welcome!
Just bear in mind that this method is destructive and it will be laborious putting things together again if you don't get it right first time.
Duplicating the original objects and using clipping masks might be preferable in some cases. Or else keep an undivided original to hand.