There are a few methods, but this is the most common. Most players that go for this tend to keep Lani at low levels and then go into the key door in the 4 star dungeon. If you haven't, you can collect the Eyes of the Unknown from the succubus dungeon to help with exp management. Once you get challenger to level 7 or 8, the high level light spells should be enough to kill the metal slimes. Just remember that the average party level is what matters, so you can still get it with Liam level 30 and lani at level 1.
The guide you have was probably written before the affection stat was added. I changed some progression relatively recently.
For magic, it's intentionally limited until the academy is added (so it's not possible to get the max title in the magician title sequence right now) . The only way to get magic points is by reading the books in the library. If you haven't, you also get them the first time you read the monster books all the way through (you get intelligence on subsequent readings).