We only had two gjallarhorns and couldn't burn him down in the first 40 seconds so we had to take 7 minutes.
Our strategy was to break bonds, run to the other side and pass the debuff around, run back to left corner and dps till mines spawned, then dps till he was in burn range and hope to god we kill him.
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It's weird that completing this activity without a particular weapon is considered an achievement. And it is which is sad. Good play though.