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If you have 45% fire defense AND healing flames, you’ll be completely safe actively (not afk) farming on a /fire scrapper. The 90% fire resistance allows all my fire farmers to skip healing flames, but if you don’t skip it, you should be covered at 75%. My Fire/Kin troller, Fire/Fire blaster, and Plant/Psy dom all do fine in the middle of the bloody rainbow map with 45% fire defense and 65-75% fire resistance, though the blaster might need a couple of small greens per run.
Not sure how the damage would play out though. I get in the zone while farming and can forget to click insps for long periods of time, so there’s that wrinkle 🙂