Give them items like Havemeal many times. Where to buy Havemeal items? I only found them at Castles Sebastian and Tower of Saints. I am sure there are more places to buy them.
About this part, different items give different amounts of loyalty. Havemeal gives one, but the FullCourse item gives 6 so if you have money to burn I would go for the FullCourse item instead.
- Before taking the Fire Palace, you need to break all crystals in the map to not have a hard time marching your troops later on.
If you want to be weird or in New Game+, there's no set order you have to do these Palaces in so you could fight it early even if it's by far the most difficult.
FirePalace grants access to some powerful items, such as the permanent class change item into Paladin and the PowerFruit item that raises a general's strength by 10. Other fruits can also be found in other locations though.
LeadershipFruit, which raises a general's permanent army count by 500 at the HermitCave.
SwiftFruit which raises a general's permanent speed stat by 10 at MiragePalace.
WisdomFruit which raises a general's permanent magic stat by 10 at DeadRuins
There's also a ManaFruit which raises a general's max mana by 50, though that caps at 999 regardless if you use it or not and I don't remember where that is.
The fruits are likely only going to be useful in New Game+ or on higher difficulties where you're in no rush as they are extremely expensive, but they can also be rarely obtained by scouting the locations where you buy them.