Boss 20 - Emeraude
Boss 21 - Dullahan
They do not have the best defense and their offense, although strong, is relatively slow and shouldn't give you problems. Just use tons of Nova-based Artes and they will die in no time.
Boss 22 - Plunsap Treant
Boss 23 - Proserpina
Final boss - Lambda Angelus
As for the fight itself you should try to remain as offensive as possible. Hit him with Nova-based skills as often as you can. When you aren't attacking, make sure you are defending and dodging. He isn't the fastest boss you have faced but he is very powerful. A key thing here is to make sure he focuses on Asbel or whomever you chose to control. If he goes after your mages, they won't survive long and you will lose a key part of your party. It is inevitable that someone will die though and when they do, don't be afraid to get them up using an Elixer. Have at least five before entering the fight so you can easily revive teammates at full health. If Asbel goes down, the fight is much easier if you can get him up with an Elixer so he has full HP to instantly engage the boss again.
Disaster Flash is his special attack during Eleth Burst and boy does it hurt. This Arte will drop anyone it hits and it has a very large area of effect. As soon as he enters Eleth Burst, make sure Asbel is the maximum distance away from him so that you do not get it. Do not be surprised if 2 or 3 of your allies go down when he uses this move. There is sadly no real way to block this. You will just have to eat the attack and hope to survive. An All-Divide would cut that damage in half but once again it will cut yours in half as well and that is not always a good thing.
Aside from Disaster Flash, he shouldn't be difficult to deal with. He hits hard but not too often and if you maintain a good offense you should keep him stunned most of the time. If things get rough, do not be afraid to use your items. Elixers, Hourglasses and Arcane Bottles are great, and this is the best times to use them.