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.
Flame thrower dwarves. Getting past even one is beyond difficult, and fighting more than one is a Luck-Based Mission.
The Ruborian Bombers are either this or Goddamn Bats, depending. Once you get to them they go down pretty quick, but until you get to them they'll be cheerfully lobbing grenades on Minion-pulverising strength at you and your non-red little guys.
The Sand Worms! It's impossible to kill them without using Blaster Beetles, they can kill dozens of minions at a time and they have incredible reach so even if you're not in the sand, you'll still likely lose a few minions to them.
Gamebreaker: In the first game, get the best equipment maxed out with only brown minions. You just turned your Overlord into an adamantium-plated Stone Wall that hits as strong as a speeding train.
Kahn, during his whole Get Back Here Boss routine, dragging out the fight with horrible dialog. He tried to do Punctuated! For! Emphasis!, but it wasn't really coming from his balls; rather it sounds like his diaphragm is where his adam's apple should be.
Sir William the Black has some pretty narmy dialogue in his own boss fight, too. But then, he is a Camp Straight whiner, so...
Refuge in Audacity: The first mission of the second game involves pitting your minions against seal-clubbing hunters because you need the life force of said baby seals while PETA-Expy hippie elves try to get both sides to stop.
Ugly Cute: The Minions. Rose and Velvet even give them the charming nicknames of "Cherubs" and "Pixies" respectively. Visually fearsome to the point of hideousness. But they make the most adorable sounds that two metal objects scraping together can evoke. To hammer the point, the minions are a cross between E.T. and the cuter moments of Sméagol. They can look downright adorable, for example with the sequence in the tutorial of the second game where the minions disguises themselves as children, then they begin to sing...