This is the strategy I used (assuming you already beat the crystal in the first round as you can't do any worthwile damage to him then):
Before the second battle begins Valkyrie will have her own weapon and come completely devoid of skills. YOU HAVE TO REEQUIP THEM!!! Make sure to equip Observation and First Aid because it will be a long fight (think about Free Item as well). The other choices are up to you. On to the battle. Your other characters are pretty much useless in this fight because there is nothing they can do. You can use them to try to get a couple of Soul Crushes in at the beginning but when his HP starts to fall to about 75%, you gotta split up. Split your party into two different teams, Valkyrie being on her own team and the others being on the blue team. Transport each character to a different "corner" of the stage and turn them back to the original blue team. After you've finished this, it should look like a cross between your four characters or there should be two characters roughly in the same place. This is where Observation and a few Foolproof Talismans and Trinkets come in. Don't use them until you see the mark for his Powerful Name spell and protect Valkyrie. From here on out you probably won't get any Soul Crushes unless you have three amazingly fast attacks on but her growing attributes should be more than a make up for it. If by chance Valkyrie does die, just get one of the spare characters (with foolproof trinket) to revive her. For this you need to make sure that all of your other characters stay alive. Toward 25% HP he will unleash his Ultimate Attack: Meteor swarm. By this time, the only one who can stand up to him is Valkyrie...everyone else will die under that kind of power. Just spend a few rounds bringin everyone back to life and using Foolproof Talismans at first sign of serious attack. After everyone's alive and well, just wail on him with Valkyrie with all you got and the battle should end shortly.
