as far as the raws goes he does not attack the kingdom!. Spoiler: a bit more background about their interactions they have a throwaway battle where the mc use his guild and resources to fight in his city (the one where one of the enemies guild gave away before disbanding and transformed in one of the main temples ) where the mc loses a lot of resources and manpower and this is only the one that we can call real fight between them because all other previous fights was more like enemy guild fighting the mc and borrowing corrupted beasts and fooder/trump cards but the beast emperor did not mainly participated in the fights he only let it use his beasts or members of said guild who he transformed in apostle fight in it , where in this fight he was active doing things like commanding the beast from the temple and even used his contribution points with the faction to unleash some trump cards against them so we can see this misdirection war being the only real fight they fought against each other. a little back info about the reason of their fight: the fight itself was a misdirection because he(the mc) wanted to destroy a pk faction teleport station and used the emperor hate and popular opinion to make everyone think he was going for a final war against him, in return makes the beast emperor and his allies like the pk faction amass their forces in the emperor's city and his surrounding as i stated before. So once his elite squad destroy the pk's faction teleport station the mc's armies start to retread , this is the last big fight they have against each other as far i remember (ps: i'm letting raws chapters build up so i have not read the last 30 chapters released in chinese so the spoilers is up to chapters to earlier 2400 chapter).
Their was different attacks that involve Beast Emperor so sorry for the minor spoilers; chapter 1063 is when Heaven Burial, Abandon Wave and some other guild(if their was one I don't remember) attack the town with the horn monster, chapter 1335 is when Abandon Wave attacks with his own hoard of monsters