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.
Demonic Spiders: In Trine 2, on Hard/Hardcore mode, pretty much every enemy is this, as befitting the difficulty. Most things kill you in one or two hits in this mode, and death is more permanent (you need to reach a new checkpoint to resurrect anyone, and then only to 50% health), so everything is innately more dangerous. Making this worse, the common goblins appear quickly, randomly, and often with little warning, especially when they come from the background behind you. Immediately upon arriving, they are ready to attack, and their aim is absolutely perfect, no matter how insane the arc to reach you is. Be prepared to die from dodging one enemy, only for another to appear and throw a spear at your position before you can land and dodge again. This doesn't even include the times where the camera is concealing a goblin just off screen, who will of course throw a weapon at the worst time, or the occasional moments where their perfect aim allows them to attack through otherwise solid platforms. When fully upgraded, you can wipe them out pretty fast, but it only takes one slip up or second of bad luck to lose a character (the fickle respawn system often means your next character will spawn next to an attacking goblin and die before you get the chance to react).
Given that the last level is only tested by 1 person outside the development team, this is a given.
Even Better Sequel: At the very least, Trine 2 is much more polished than it's predecessor.
Game Breaker: Zoya breaks combat in the original once she's shooting three arrows at a time [four with a certain treasure]. Give her the poison vial and each of those is doing double the damage. She can take down most bosses in two hits. Once Amadeus gets the ability to summon 5 or more planks, 6 or more boxes, and a flying wooden block for Zoya to swing from he breaks puzzles as much as Zoya breaks combat.
Trine 2 balances combat by making the goblins much faster than the skeletons ever were (giving players not enough time to fully charge Zoya's arrows) and taking away Zoya's Spread Shot capability, making Pontius much more preferable when mobs of enemies are involved. Amadeus got a hard limit of four summoned items regardless of type and his floating platform taken away, but he still can blow through the puzzles when fully upgraded.
Zoya's low gravity arrows in the second game, coupled with Amadeus's boxes can pretty much get you through the entire game once you get the timing down.
Goddamn Bats: Literally. Despite being one of the rarer enemies in the game, bat swarms slowly chip away at your health and move too quickly to be easily hit with a sword or bow. They'll also frequently fly offscreen and then come back to torment you again a few seconds later. Fortunately, Zoya should have the triple-arrows (or exploding arrows, which requires maxing out your skill with Arrows on Fire) by the time they show up, which makes short work in a few clicks. Without her, however..
Scrappy Mechanic: The physics are simultaneously the most fun and the most frustrating thing about the game. It can be nice to just drop a block on foes to kill them, but around the point where you miss a jump and get hurt for the sixth time because the engine refuses to acknowledge that your character has reached the ledge, it starts to wear a bit thin.