The Master of Engines is a mission in B.T.D.6 that has a 4-4 Dart Monkey at the end of the first straight on Ad Astra. It is insanely powerful, as it fires three spiked balls with a popping power of 500, and has a powered up Super Monkey Fan Club Ability (named Brotherhood of the Super Monkey Fans), affecting every Dart Monkey on the screen, converting them into 0-2 Super Monkeys, and it lasts for 45 Seconds. Also, the spikes do 30x Damage to M.O.A.B.-Class Bloons! However, he requires metal, so he must take the thing with the most metal (i.e. your most expensive tower) every three rounds. However, as Spike Factories, Monkey Aces, and Cannons (Bomb Towers) have more metal than Super Monkeys (unless it is 2-4), it will take those first. You can't use Boomerang Throwers or Pineapples on this mission, because they don't have metal, and you can only use Agents that have metal (So, no Monkey Storms, Squirrels, Beekeepers, Pontoons, Bloonberry Bushes, Portable Lakes, or any agent that has no metal). Every bloon is +2 ranks, and you must beat it on Easy.
Oddly, you can use Monkey Apprentices here, but it isn't so odd until you consider that spells use metals (in this case, combat and empowering, it is iron).
Apparently, the spears used by Tribal Turtles have steel points. Where they got the steel is beyond me...