How to open stuck fuel locking ring off fuel pump assembly?

My 2001 Deville has an accessible fuel pump underneath the trunk. However, the original locking ring is made of plastic. I tried everything possible to unlock it by rotating it, but the plastic is soft and very brittle. I don t want to take the tank down. All I want to do is test the fuel level sending unit and... show more My 2001 Deville has an accessible fuel pump underneath the trunk. However, the original locking ring is made of plastic. I tried everything possible to unlock it by rotating it, but the plastic is soft and very brittle. I don t want to take the tank down. All I want to do is test the fuel level sending unit and most likely replace it. I tried the brass punch. It won t budge.
Any other options?
Update: No tool fits the staggered rings. This is an older type of ring.
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