You have to be a very powerful warlord once you take on being the king. And when I say very powerful, I mean ~50 juggernauts/twilight knights + same amount of imperial sentinels + another 200 troops for the grinder. Aside from that, make sure you have at least half a million in cash. Once you start the conquest, eliminate, or almost eliminate one kingdom completely. Best start when as many factions are at war as possible. Make sure you are at least level 22-25, with a solid foundation in whatever spec you want to play as.
The most effective tactic against invading factions is to be extremely aggressive with them. Catch them during a siege on some castle, and help that castle. Catch them in a battle against some lord, and help him. Try to get as many lords defeated at once as possible. Immediately try to capture at least several castles.
This is how I defeated kolkar in their 1st week. I caught them fighting with swadia, around 1k swadia on 1.5k kolkar troops. Tactical advantage - infinite. Despite extremely OP troops on their side, me and swadians steamrolled them with heavy cavalry.
In terms of difficulty, whom to wage war on (from hardest to easiest, also SUBJECTIVE depends on your playstyle):
Koklar
Swadia
Sarrannid
Crusaders
Vaegir
Balion
Everyone in this game is garbage, and can be disregarded.
