Here's the combo that'll get you the win on Hollow Vergil, no matter the phase or the difficulty mode. Practice this a bit in training mode to get a feel for it. The initial move does have some i-frames (one of the few moves that do, along with Atomic and Perfect Slice). Don't be afraid to walk up to Vergil and charge. He wins if you let him play. Don't let him play.
Charge LT + B* -----> Y Y -----> LT + Y -----> RT + B -----> RT + B, then run back to Hollow Vergil and repeat.
* This must be SOLAR FLARE, NOT Rising Star. The timing for this move is, when charging, right when your sword would go to flash signaling you're charged is when you want to release, not after. Try out Rising Star... charge it until you see your sword glow, then try and pull off a Solar Flare. You'll see the difference in the moves. The timing is tight but it'll juggle Hollow Vergil if it hits no matter what he's doing, and the range is crazy far.
Got this technique from a no-damage video after struggling a bit. They use different strategies between rounds sometimes but this combo seems the most consistent.