It was recently mentioned that the Monstrous Crab is generally capable of wiping out even a powerful 3rd level party. It seems to me that, lacking any anti-ranged capability whatsoever, it is trivially dealt with even by incredibly weak characters simply by staying out of melee with it, which itself seems fairly simple for an organized party of level 3 characters. So what makes the crab infamous for character-death-causing?
Make It Harder
This is definitely something I've had happen. I wrote a whole blog post about the exact same thing - while GMing a fourth level Pathfinder party, I found that I had to make bosses eighth level to challenge the PCs. So you're probably going to need to up your CR/EL expectations. Pathfinder PCs have higher damage output therefore old 3.5e wisdom is somewhat out of whack (same with 3.0 to 3.5 - in general for 3.0 CR X you need 3.5 CR+1 and Pathfinder CR+2). You can see this in the encounters in Pathfinder Adventure Paths as well, the CR of capstone encounters is often like APL+5. So I'm afraid you're going to have to go for some "harder." Also, solo opponents in Pathfinder are meat for the beast because it allows the PCs to exert tactics without resistance; give bosses some help even if they are just a wad of mooks.
Now you can also push opponent effectiveness by improving their tactics and not just beefing them up - but I already do this, and it doesn't completely close the gap. But yes, try to come up with effective tactics for the bad guys, especially if they are on their home turf. No one plays too fair when their life is on the line. Random orcs aren't going to be brilliant but they don't just charge lemminglike into the blender either. Look at their preparations from the point of view of "if I were doing this to be effective." I like recruiting actual people to play main bad guys - they are a good 30% more effective than when your attention is split between running them and doing all your other work. Try some of the player-oriented tactical advice from this question on your NPCs/monsters. In a wilderness setting, people who have any warning will use terrain to their advantage - "Shoot at them from up in the trees," "Get in the canyon so they don't see us from far away," or whatever.
Also if you're not having many fights a day, consider that challenge doesn't have to be all combat. What is the party's goal? Unless it's "harvest souls because we are serial killers," it's likely their goals involve finding things, saving things, building things, helping people, or whatnot where the challenge is more skill and roleplay oriented and not solved by killing. For example, in the Kingmaker adventure path, the PCs are in the wilderness trying to build a kingdom, and if they slaughter everyone that faces them then they don't have much of a kingdom once the fights are over!
Lions and Tigers and Bears... Oh my!
I think the number of dangerous creatures out in the wilderness is almost innumerable, so I will just list categories with a few examples.
- Snakes (Venomous or large)
- Poisonous frogs
- Mosquitoes (with or without diseases)
- Army ants
- Bees and Hornets
- Large Mammals
- Large Cats (Lions, Bobcats, Lynx)
- Large Canines (Wolves, Hyenas, Jackals)
- Others (Boars, Bears, Stampede of Buffalo)
- Physical hazards
- False grounds / Pits
- sudden cliffs/waterfalls
- Falling obstacles
- Wild fires
- Flash Floods
- Water Creatures
- diseased water (not boiled)