Can I save this Euphorbia Ingens/should i decapitate?

[Cactusfrend]

Hello,
I have a Euphorbia Ingens which had developed some dark brown marking towards the tip about last week. I have kept an eye on it for the past couple days and have been putting an at-home fungicide on the markings. It does appear as though the color change is spreading, although perhaps not that quickly and no longer quite as dark as it was (Attached are photos). Given the current situation would it be best to proceed with surgery and removing infected parts? If, so what is proper procedure and future care. Or should I continue to monitor and hope that the cactus is able to compartmentalize the issue?

Thanks!