Removing Obstinate Cassette Locking Ring.

[boydj]

No, it's small slices removed from the side of some teeth. Comparing it to my new cassette it's definitely damaged.

That's by design to assist shifting.

What I do to remove the cassette lockring is to put the tool in a vice and then put the wheel onto the tool and use the chain whip while holding the wheel for extra leverage - never fails.

 

[Ajax Bay]

The chain was slipping and it was the second chain fitted to this cassette. I can see signs of wear on the cassette on some gears. Some of the teeth had obvious "irregularities". . . . . . No, it's small slices removed from the side of some teeth. Comparing it to my new cassette it's definitely damaged.

That's by design to assist shifting.

This ^ ^ . You can't tell by inspection if a cassette is 'too worn'. But the OP's said theirs was slipping before they fitted the new chain.