Agnus' Room/The Gauntlet
Take all Red Orbs and leave. In the cage you will be attacked by a new enemy - Faust. He's accompanied by two Mephistos. Use Charged Shot on them - there is no better weapon than Snatch/Charged Shot combination. In the next room there are some lasers to avoid - slow time and quickly run dodging rays. Activate the machine and return. Go back to Key Chamber.
Key Chamber/Grand Hall
Some more knights to dispose of - this fight can be very hard but you can slow time here. Go to Grand Hall and go to the entrance to the south.
There is a very nasty trap here. Try to avoid laser rays jumping. At the end of this room you can find a Secret Mission 06. In the room with gigantic blades, fall down and destroy Mephistos. Activate Key of Cronos and jump up on blades to reach door.
Secret Mission 06: Vermifuge
Destroy all Chimeras before they kill Scarecrows.
Tactics: Take Scarecrow to the safe place using buster, then destroy Chimeras.
Experimental Disposal/Meeting Room
Destroy a big group of knights and break a statue of Red Orbs. Above the pole near elevator a Blue Orb Fragment floats. On the second floor enter the room and fight Alto Angelos. This is harsh war so use Devil Trigger if necessary.
Secret Mission 07: Free Running
Go over the bridge.
Tactics: Shoot to Mephistos with Charge Shot 3.
Dante could be damn hard to beat but he has a weak point. Jump and then use Snatch on him (2nd or 3rd version) with instant Buster.