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: The snipers in Chain Reaction. They can take you down from full health and armor in about 4 hits, often have Improbable Aiming Skills (hitting you from across the level while blindfiring from cover for instance), and are very difficult to find and kill without memorizing the level or using the upgraded camera. When climbing across wires, be sure there are none of these guys around.
The mines are easy enough to avoid if you just gun it straight through with only slight variations in pitch. The submarine at the end, on the other hand, is a doozy.
Easy fix: Be extra-careful with your remote torpedoes throughout, and at the end, stay at a safe distance and wale on the submarine with them. CAUTION: DO NOT—DO NOT—SET THE MINES FIRST. If you set the mines, you will go into "timed escape" mode.
Island Infiltration isn't a cakewalk either no matter what; enemies are armed to the teeth with enough weapons to tear your armor to pieces in seconds. That's not even getting into the rail shooter sections where you only have one set of armor and that's it.