I know that this game is kinda old, but I'm still curious about what are the most effective strategies.
One I know is Flash Fire and spam Fire-attack moves on it, but what else?
I play Pokemom Conquest quite a lot and my strategy almost always work.
I normally try to take as many warlords with their kingdom before July as that's when the others attack. If you don't have a strong army by that point you may struggle.
Now there is to ways you could do now.
Storm attack: Go all out offence making sure you take down at least 2 kingdoms per month. I recommend getting two armies. But be warned as if you don't take at least 13 kingdoms in the one year period you have a chance to get an event where enemies Pokemon get a lot stronger. There is an other event when all the kingdoms turn against you but you don't have to worry about that event as strength does not increase in the end.
Defence mode: After getting about 5 or so kingdoms this is when you want to do this strategy. Make sure your strongest army locks other kingdoms path. Basically wait now until there is only 1 army left then go to war.
These are my strategies but if you don't like waiting use strategy 1.