To beat his castle you need angels. For that you need to max out all 3 races use the orc castle (recommenned) he itself is immune to the angels freece an heals from it. so use dragons and titans. Do not send normal units then the boss wont attack you special units!
He has no sword, he pounds the floor damaging your castle by 500 each time, all you need is ONE Angel Knight to stop enemy units, as much dragons and only the ELF GUARDIAN to attack, others won't do anything - Be the elves and use this simple method and you should be fine (The Angel Knight thankfully pays back his damage using a sword slash
