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Rattlegore will gain a stack of Rusting for each successful melee hit. This will increase his damage by 25% but decrease his attack and movement speeds by 5%. The achievement is failed when he reaches 6 stacks.

A group with another class that can taunt will have an easy time, simply taunt him back and forth while the dps kill him.

Guardian Druids can easily kite him too, using Cat Form, Stampeding Roar and Dash, and can also make use of Savage Defense to decrease the chances of getting more stacks, or even allowing them to fall off (dodged strikes will not apply the Rusting buff to Rattlegore).