For alloy cartridge lockrings, I use a 1-1/4 holesaw to cut through the ring down to the cartridge, then punch out the cartridge. You need to improvise a guide for the holesaw. I use a small homemade adapter so the spindle becomes the guide and power through.
After the cartridge is out, you can try to turn out the lockring, or carefully saw it radially, and break it out one section at a time, or file it down to the roots of the thread (crest of the BB shell threads) and use a tap to clean it up.
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