Have done Flame Leviathan, Ignis, Razorscale, XT, Kologarn and Freya in 25 as ret, though this was before the nerf to Seal of Insight. Not sure which ones are still doable. This was in around 485 gear.
Another question: I tried doing the Hour of Twilight 5-man heroics last night, and couldn't get past the tentacle trash before Benedictus. Anyone done that, and got tips?