The point is, this was the fifth, extremely hidden and not announced difficulty. You just had to experience it and know about it to truly know what you were getting into should you then select it.
4e DnD actually works this way. You don't realize how awful things are until you really experience them. The snake fight is a good example of this, so is any fight with stun (save ends). However, the secret difficulty setting is really simple, adding soldiers. Do you want your fight to be a lot more difficult with little or no change in level and xp reward? Add soldiers! High defenses, high hit bonuses, good damage and usually awful conditions! Woo, welcome to soldiers! Equal level soliders are a pretty good challenge, elite or not. They are difficult and if there are a lot of them, yikes. Lower level soldiers are still a good challenge, did I mention the high everything? In your xp budget, you can actually use more of these lower guys too, making things both harder and easier at the same time. Higher level soldiers? Ugh. Just ugh. Even if you have good feats and good equipment, it's going to be a really tough time hitting these guys.
Now, do you necessarily need to do something about this? Not really. That's just how the game is set up. Just try and be cognizant of it when you are budgeting your fights. Soldiers are worth more than their counterparts, almost every time.
Are there things you can do? Sure, in a game I ran, I chose to give the players a way to lower defenses and increase damage taken on the showcased soldier NPC. It worked out alright. If I had to do it again, I would probably have it decrease defenses a little less, but it made the showcased items a commodity that was carefully used throughout the fight. That being said, do you need to do that if you are actually staying within xp budget (hint: I did these things because I specifically was NOT)? No, not at all. It's just something to keep in mind when setting up fights.