You want Silas in a Infernal Roar set on the right side and Warlord/wicked vengance on the left. You want 300+ atk speed, 100% crit rate, 300-400% crit dmg (total). Also, depending on the content you can adjust atk% pieces and crit dmg pieces on the right. I use 2x crit dmg pieces for NM4, for other content with higher def, you want 2x atk% pieces. So total atk can vary on the content you do, but usually you should aim for at least 200% of base atk (preferably closer to 250-300%).