To get this achievement, you have to "kill" the 12 standard monsters, then the final boss.
The twelve monsters are:
Spider, Troll, Imp, Goblin (found in the Cave)
Ogre, Bandit, Wolf, Ent (found in the Forest)
Rat, Ghost, Skeleton, Vampire (found in the Mill)
CAVE
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To enter the cave and fight a monster, you must flip the ball into the "Cave" hole twice. You can see the cave hole on the right hand side, about halfway up the table. This is the hardest monster area to access, so if you're planning on getting this achievement, I would try to get all four monsters from this area as quickly as possible, preferably with the first ball.
The best way I found to get into this hole is to hold the ball in the left flipper, then release and hit it near the tip of the flipper. Be prepared to get frustrated.
Once you manage to hit this hole twice, you can select the monster to fight with the flippers and the launch button.
MILL
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To enter the mill, you have to get the ball into the water wheel (top central part of the table), then perform the skill shot with the top-left flipper. Once inside the mill, you can select the monster to fight by "poking" the ball into the appropriate hole (use either flipper button to do this). From top to bottom, the holes correspond to the Rat, Ghost, Skeleton and Vampire, so try to hit one you haven't fought yet. Annoyingly, the ball will sometimes roll over the hole and into the next one, especially if it is moving at speed.
FOREST
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Easiest monster area to access. You just need to get the ball into the bumper area, usually accessed by the lane that's inbetween the waterwheel and the sword ramp. Once you hit the bumpers 15 times, the next time the ball falls into the hole below the bumpers, you can select the monster to fight.
FIGHTS
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To fight the monsters, just hit the sword lane and the "smash!" target. Extra damage can be attained by hitting the shield lane immediately before the sword lane (this is a "counter attack"). There is a clock running, and every ten seconds, the monster will hit you back. You can reset this clock to 10 by getting the outer orbit lane (around the top), or gain a second per bumper hit. Also, if you have obtained any magic scrolls, you can try to get into the mill to cast them, but I would save them for the final boss. Whatever you choose to do, basic monster fights aren't usually much of a problem.
SPECIAL MODES
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If you can hit the sword lane five times in a row, you'll get the two-ball skeleton mode. Hitting the various lanes will "kill" skeletons and gain points. At the end of this mode, if you have killed enough skeletons, you'll get the Skeleton monster trophy, and won't have to get it from the Mill.
Also, if you can hit the Shield lane three times, you'll get Bat Frenzy mode. The same applies, if you can "kill" enough bats, you'll get the Vampire trophy.
TRAVELLING
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In normal play, getting the outer orbit (the loop around the top of the table) gets you a "Travelling" points bonus. Hit a few of these in a row, and you'll get a special reward. Sometimes the reward message will say "Found {monster}", which automatically starts a fight against the randomly-named monster. If you're lucky, you can also get a Travelling reward of a monster trophy, without needing to fight it.
FINAL BOSS
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Once you've got all 12 monster trophies, hitting the ball into the hole below the bumpers will start the final fight. This is just the same as all other fights, but the boss has a lot more energy, and is generally resistant to single-sword-lane attacks. The best approach is to use your spells, which you should have saved for this mode. Use a few spells, and hopefully get some smashes and counter-attacks, and he should crumble, and 50 millions points (and more importantly, this achievement) will be yours.
Be careful, as two hits from this enemy is usually enough to "defeat" you, and this results in three randomly-chosen monster trophies being removed, and you'll have to fight them again to access the boss mode.
POTIONS
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If you're finding fights tricky, entering the Cave during a fight will use a Healing Potion, restoring your energy, but this generally happens by accident, as the hole is tough to hit on purpose.
Good luck! I reached the final boss four times before I killed him. Happy adventuring!