Once you get it started by hitting with the screwdriver you need to twist it or knock it forth and back a few times to loosen it up. If you have a good pair of Diagonal Pliers or a pair of BX Cutters you can sometimes get in there and cut one or more of those welds or the ring itself. Just don't try to get it real fast. Be patient and work it out. You will feel the knockouts get looser as you move them forth and back.

For subsequent rings sometimes you can use your pliers to carefully pry the ring halves until they are up enough to squeeze them together and then forth back and back parallel to the anchor point until it breaks off. If you try to rock in the other direction it will tend to twist the next ring up. Just keep removing the rings in the direction that they were punched and you should be fine.

In a situation where the electrical sub-panel is recessed in a wall, you can use a long thin screwdriver and stick it between the edge of the wall and the edge of the panel to apply a little pressure on one of the rings from the outside to get it started. 

Each electrical sub-panel manufacturer's concentric knockouts are different in strength and ease of removal.  Sometimes they pop out too easily and you have a much bigger opening then you need.  Fortunately there are Reducing Washers available in all trade sizes, made to transition from the larger opening down to the size that you need.  If you punch out the wrong knockout, it can be sealed up again using a Knockout Seal.  Both of these items can be purchased online or at almost any local electrical supply company.