I think that Ability is the last Overpowered Thing on a Marauder. Let them have their Damage and fancy Cooldowns but having the Highest Damagereduction of any Dps for such prolonged Time isnt balanced. 30 Seconds is alot longer then any other Defensive Cooldown, its puts Rage Marauder at a 53!!! % Damagereduction. Thats more then any Tank while having no Damage Penaltys at all, in fact Cloak of Pain deals Damage.
If not a complete Redesign i would say a reduction of - 5% magnitude and -10 Second Duration would be a good Start.
You know, when you whine about something on a Marauder, theyll change something highly unrelevant, like this time, Ahni nerf whereas Rage was complained about.
Incomming Ahnilate base damage nerf.
Besides CoP is easy to counter..l2 use roots or stuns if it bothers you