I was dual wielding two Mercy daggers. It deals bonus damage equal to 3% of the missing health of the monster. If you´re fighting a monster with about a billion health.. this bonus damage will grow exponentially as you keep hacking.
The character (orc marauder) was built especially for this fight. Most one-shot abilities (with a high cooldown) are useless, because the fight lasts extremely long. Many damage abilities are useless as well. But speed is good, extra hits are good. And you need all the anti-resistance abilities that you can get, because Amakthel has 90% all resistance and 90% physical resistance, too.
So I maxed every talent that helps here and can be sustained indefinitely, like Momentum, Bloodbath to sustain it, Exploit Weakness to lower resistance, Bloody Butcher to lower it further, Greater Weapon Focus for more hits. Blinding Speed is great too but it has a very long cooldown, so I used a ring instead and maxed charm mastery.
If you're interested, here is a link to the character.
http://te4.org/characters/150033/tome/1 ... 0fc0515847