OK, so I just unlocked this a few hours ago, and I just wanted to tell you guys what ended up finally working for me. I only had one troll bomb, so I took out the one on the left(For me, it would always do that one first)... The one on the right stayed up no matter how many times it got hit. Twice, I got screwed by this, as I was on my last ball and got it stuck behind that troll. However, I had all 4 balls when they popped up, and I got 2 stuck behind the right troll, and kept shooting with the remaining ball. After hitting the gate 7 times (after making all lit shots once...including the catapult, left orbit, left ramp, ball lock, right ramp, right orbit) it opens up, and this goal is yours! Almost had a heart attack lol. Good luck everyone!
To get to battle for the Kingdom, you must first complete....
3 Peasant Riots (Shoot the left ramp a total of 12 times)
3 Damsel Rescues (12 shots to the right ramp)
3 Joust Victories (12 shots around the left OR right orbit/loop)
Catapult Ace (12 shots to the catapult)
Master of Trolls- You have to kill 10 trolls. You get 2 when you take them out originally, then you can get the rest during Troll multiball maddness, you can also get them randomly through Merlin's saucer with "Whack a Troll".
Last thing you need is Castle Crusher which means taking out all 6 castles. This is a TON of shots. It takes one shot for the first drawbridge, then 2 shots for the gate, then you shoot it through the castle to take it out. Each castle you take out, the next requires one more shot per stage (drawbridge, gate, castle). So that makes it a grand total of 54 shots to the castle to destroy all 6 castles. These can all be done in any order, but you must finish them all to even attempt Battle for the kingdom. I'm getting real close, but holy crap, this is difficult. Make sure and take out the first 2 castles as fast as you can to get your extra ball. After that, work on getting each item three times. Then make every hurry up award you can because at 10 you get an extra ball, and then at 40. Once you complete at least 1 Damsel save, peasant revolt, catapult ace, joust victory, and troll victory you get a chance at ROYAL MADDNESS. This gives you 20 seconds to hit 5 shots twice. You have to hit the left orbit, let ramp, right ramp, right orbit and catapult 2 times each. Each time you make one shot, the timer resets. If you complete all shots in time, you get an extra ball. GOOOOOOOOOD LUCK!
EDIT* I just got to the Battle for the Kingdom last night....and messed it up. My strategy is to take out 2 castles. Quickly done by hitting the castle, and when the ball returns, shake the table right(right on left stick) and hold the left flipper up. It will land and stop, and you can just keep doing it over and over. Once I take out the 2 castles, collect your extra ball by hitting Merlin's saucer. Then I will go for Peasant's and Damsels. With some practice, you should be able to criss cross the ramps until you collect one of them. When the ball returns, hit the other, then rinse and repeat. Go for multiball, and use this time to get many castle hits. You can take out at least one castle if you take your time. After that, finish your peasants and damsels, then work on the catapult and joust. Get trolls whenever, and take out the remaining trolls on troll maddness. Then just clean up the remaining castles using the above method. Once complete, start the battle for the kingdom, and hold your breath. Just don't get flustered, take your time and try to keep as many balls in play as you can until done.