These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Game Breaker: Missiler Bots. They function like Cattails from Plants vs. Zombies, which means that they can attack enemies anywhere on the screen! Also, unlike cattail spikes, their missiles find another enemy if their current target is destroyed! The upgrade makes them fire missiles even faster! One can build nothing but these, Fusors, and anti-stealth units to solo many stages.
That One Attack: Destroyer's Mouth Beam, which kills 2-3 rows of units very quickly. It's like Dr Zomboss' Winnebago attack combined with his fire/ice balls, but unlike Zomboss' very predictable attacks, Destroyer's beam comes out from nowhere.
You have a slot machine, which you have to time correctly in order to get your Minirobots. In order to get three in one go (which is essential here), you'll need to match all three. You can't afford to take too long timing the slots or else the machines WILL overwhelm you.
The enemies fall down from the sky via Meteors. Not only can they randomly fall around the middle of the area, allowing the enemy troops to get behind your front lines, they also happen to be One Hit Kills on your units (that includes both defense-based units you can get here). If a whole bunch of them fall on your middle area, tough luck!
Knights. When coming from the meteors dropping in the middle of your troops, these guys will easily take out your back rows due to their piercing attack ability. Worse still, these have quite a bit of health, hurt a lot (on both your Minirobots and your health), AND are fast!