And 6 seconds is a lot shorter than any other defensive cooldown, and that's how long it lasts if you don't hit them.
So, the method of dealing with a Marauder is:
1) Don't put up ANY dots before Cloak of Pain.
2) Run away and don't even attempt to fight during Cloak of Pain. Most stuns and roots deal damage so they're not applicable. Some AoE hits the Marauder? Ooops, bad luck.
3) Save a knockback for Ravage.
4) Save a stun for Undying Rage.
5) Save your rage and anger for Force Camouflage.
6) If he happens to have Saber Ward available, just laugh at the sheer pointlessness of it all.
7) If you happen to be a class with high-hitting white damage attacks, don't use them during Obfuscate/Pacify. Just stand there like a lame duck. Or run away.
At this point, you're most likely dead anyway but at least you managed to avoid extending the uptime of Cloak of Pain. Well done!