You can start and hold a kingdom at day 1 if you want. I think the looter 9 guy is rubbish, instead I stack charisma/leadership for troop volume.
Step 1:
Empire takes laria.
Step 2:
Sarleon retakes laria. (Factions will re-take most recently captured town)
Step 3: ****
Take laria. It should only have 73-135 defenders. Your high charisma/adventures/barclay should have no trouble.
Step 4:
You are now enemies with sarleon, send goodwill to the empire. They are your friends.
<-->alternatly, you can pledge to the empire keeping laria and associated towns and the empire will help when laria is re-sieged.
***To prevent laira from being re-sieged (or to reduce the chances) wait 1-2 days for empire to assault a sarleon territory, or for sarleon to attack something. This makes sarleon forget about losing laria and put their focus elsewhere.
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In my most recent 3.204 game, i spent about 100 days running around as a factionless rapist. I'd join in the battle on any winning side, loot all, capture all, sell prisoners then bribe my relations back to 0. Release every lord you come across except for kings. Join in any siege defence you can as it's the best way to gain XP/proficiencies.
Once random lords you've never met get 85 or higher. Start at step 1.
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Step 5
Capture sarleon lords that are 85 or higher. Accept their oath and grant them fiefs associated with their previous town for 100 rep. I find at 100 rep they dont lose rep when you assign fiefs to other lords. You should be able to take 4-5 sarleon castles this way.
Step 6
If you arent super confident at faceraping, consider territory. To get another king to recognise you, you can't possess their lands and you must have a mutual war. This will allow you to send peace offerings and reduce the likelyhood of random factions declaring war on you.
If you find it too hard, pledge to the empire as suggested above. Try to get the marshalship, stomp every other faction keeping towns to yourself, then when theres only one other faction left, start asking for everything until you get the rebel option. Then go to step 5 and win the game.
Joeman's post is exactly the same as mine with the exception of lord relations. Devoted lords are better behaved lords.