That's why you need the 20% boost from Infiltration.
Your 30 soldiers from Gautinguk are not enough to beat the 40 from Oryeonguk without the boost.
Like
ElAguila said, make sure all of your troops are in your Main Army, before launching the attack.
They cannot be used to attack Fief B, if they are currently defending Fief A. When you have more fiefs, you will need to wait a couple of days for troops to transfer.
If you lose the boost then you'll need to hit and run.
Their soldiers won't respawn, yours will. So hit them, then retreat, wait for your soldiers to be replaced, then hit them again.
Takes a bit longer, but you'll eventually have more soildiers than them and win.