Ended up beating dmd mode today, used a lot of vital stars though. Figured I'd post my experiences with 19 and 20. Guess I can face roll easy mode with super dante for the fun of it. XD
Mission 19 was about as bad as I figured it'd be, the normal enemies aren't too bad on lower difficulties, but mission 19 is a very gimmicky mission in a pretty annoying way.
First area is a straight fight and one of the better areas of the mission, 2nd room isn't terrible either. 3rd room is evil, if you don't kill all the enemies in time, they reset and you have to kill them all again and for DMD that's not the easiest task. Involves getting them down to one bar, so they're all still out for your blood, then you can start killing them since they get a massive health boost on triggering.
The boss is nightmarish in melee range, very capable of outright killing you if you try, so it's slower but far safer to just stand back and shoot him to death. Quicksilver let me get some melee hits in, second half of the fight where the player is gimped pretty badly I just shot and hoped Vergil helped. Needless to say, I was very happy to have this done with when I beat it.
Mission 20 is pretty tough, though not nearly as bad as mission 19 as it's just a one on one fight. That's not to say it's a fair fight, DMD mode after all, Vergil is very capable of killing you outright in one combo that's inescapable if you don't dodge the first hit, he recovers health fairly fast and uses it almost constantly during the fight.
While he regens health, it isn't wise to approach him since he doesn't get stunned while triggered and can rip your face off if you try. Quicksilver helps a ton since it allows you to force him out of trigger if he sheathes his sword or does his sword orb move. Quicksilver is also near essential for handling the phantom swords asap.
Speaking of the swords, they are very nasty in this fight. If possible it's best to quicksilver and destroy them as soon as you can, otherwise vergil basically gets a free full life combo on you unless you use items to heal. Full life combo basically happens when he triggers and does repeated downward slashes with the swords out, the phantom swords and downward slashes can stun lock you very well, it's insanely difficult to dodge while he has his sword tutu of death because of how far out they reach and he teleports to directly above your current position.
All that said, still a pretty fun fight because he does give you openings, just can't go overboard on attacking him normally, as tempting as he makes it with healing himself almost the entire fight.
put the missions in spoilers since they ended up kind of wall o' textish. XD