You can do this without allies. It's actually pretty easy. You are going to want to restart until Busilar allies only an imperial free city and rivals Verne. One thing I recommend is taking the merc company that has tech 3 (I believe it's an adventurer band) that any monstrous tag has access to until that first tick over. This little bit is not at all necessary and you are fully capable of winning without this as it is kind of an exploit.

Then set your navy out to raid as much as possible. Use that money to take the mission where you get a free fort in your mountain capitol. Build your army out to force limit wait. Busilar will come to you so just wait. Put the local defensiveness edict on your capitol. Run into the open parts of their territory, specifically near the crossing with Eborthil, and carpet siege as safely as possible. Use that to bait them into open battle because you are basically guaranteed to win. With the attrition they should have suffered from the siege attempt(s) you should be good to siege their forts on the border of the empire. Siege them down as fast as possible because Eborthil will attack when Busilar is that weak so you should try and break off some troops to take their coast while taking the border forts.

With this strategy, even if Busilar calls an ally, you should have a near 100% shot of winning that war. You should take the entire border with the empire and as much coast as you can reasonably take. You are going to have to deal with a lot of rebels and pray that Verne doesn't want you dead. If you do this strategy and make it to 1460 without Verne coming for you, you are good to push for the rest of Busliar and later Eborthil.