Baal can be persuaded to join the party when, after you have beaten the game, you return to Mt. Palo and talk to him there.
Beat Devil easily
On Mt. Palo, there is a creature named Devil, who you must defeat to rescue the kidnapped children. Since Devil can be a pain sometimes, you may want to use this. Recruit a Tanki, because he is very good at recruiting other monsters. Recruit some rare creatures while walking around on Mt. Palo and then go face Devil. You want to attack him, DO NOT USE MAGIC, he is not the least bit vulnerable to it. Have a full party of your best monsters and have good defenses, if you want to beat him.
Cerberus joins Team
First, go to the Inn Northeast of the Shrine of Hypnos an the Monster nest (on the island with Jalawan, Ramuh, and the Tower of Megan on the other side of the mountains). From the inn, walk East (to your left) until you are below a space with no tree on it. Walk into the space, and you will be in the cave of Cerberus. There is a creature inside that looks like a lion. This is him. If you talk to him, he will fight you. If you beat him, he will join your party.
Get Asura, Pazuzu, and Loki
To get Asura, Pazuzu, and Loki you have to combine these:
Suzaku + Kali = Asura
Zenon + Shiva = Pazuzu
Zenon + Jinn = Loki
If you want the best of these monsters, go for Asura. He has 1,800 HP, which is 80 more than Pazuzu and 100 more than Loki. All of these are better than Dragon, Solion, Amon, and Kirin.
Get Dragon, Kirin and Amon
To get Dragon, Kirin, and Amon, you must first find 3 bones of each character. One Dragon bone is located in the Tower of Magan, another in the Zord Castle, and the final is in the Sewers of Atlantis. One Kirin bone is located in Zord Nest, and the other two are located in Zord Castle. One Amon bone is located in the Cave of Cerubos, and the other two are found in Zord HQ. Updates on the exact locations will follow.
Lucifer joins Team
To persuade Lucifer to join you, first beat the game. Then return to the Cave of Oasis and talk to him there.
Michael joins Team
After you beat Lucifer, go back to Nova. There talk to Uncle Hata and he will join you. It turns out that he is Michael.
Monster Upgrade
Make Better Monsters You can upgrade just about every monsterit . The best way to do that is to use Lich. Yep, 12 HP level one Lich. You will need to catch an Owl and Vepal (There are other ways of making Lich, but I found this to be the easiest) outside (and inside) the Tower of Magan. Then, choose the monster you want to make better and combine it with Lich. If your level is high enough, you'll get a killer monster in return!
Secret cave
In the world with the Luciferium, go to the northwest corner on foot. Explore the mountainous region to find a hidden cave. In it is the Omega Armor and Omega Sword(for Uranus) plus the Alpha Mail and Alpha Sword(for Kishe).
Solion joins Team
After getting Cereberus and Orthrus talk to all the Monsters in Nova. One of them will fuse the monster into the more powerful Solion.
Talk your way out of battles
If you go into battle and you fight a monster you have in your party (he doesn't have to be summoned) and start to talk to it. The monster will walk away.
Vaerial joins Team
To persuade Vaerial to join your party, first beat the game. Then return, traveling through the Nest of Zord, and talk to the monster on the former battleground.