There need to be a cooldown on when the CPU can blow the horn, or whatever he is doing. I got an enemy castle below 50% HP, he blew the horn. 5 seconds later, he blew the horn, 5 seconds later, he blew the horn. 3 times in under 12 seconds. I was swamped with enemies.
If you get pushed back at that point (which is quite easy to have happen), you WILL loose. There is no recovering from 10 on 1 odds.
Save up rage on his first form, then use it on his 2nd. It's all patterns recognition, and shot and jump accuracy. You can do it!