well, that my friend is one of the many struggles of the GM!! lol I'm my experience without characters being maximized (18 or 20 of main stats and 18s, or 16s on secondary stats) level appropriate monsters and challenges are Actually a challenge. Everyone wants to have those Epic High fantasy stats, but when the monsters are designed for characters with lower(more realistic stats) the challenge ratings are not accurate. forcing you, as the gm, to throw stronger creatures and encounters that can one hit your PCs just to give them a challenge with a real sence of danger. Also there are a lot of things you can throw at PCs to hinder them, rather than just plain damage, which in turn can make seemingly standard encounters that much more of a challenge when a PC is suffering from something else. Designing encounters that are very difficult For PCs, that if your players face it head on, would not normally succeed, forcing players to maybe go about it a different way, or wait until there are different circumstances. As long as you dont baby PCs, and let them know you wont save them if they get themselves into a horrible situation because of reckless, and outright stupid decisions, they tend to make better choices as far as how to handle encounters. In the end there is no real way to perfectly plan an encounter to be just hard enough without killing your PCs, you just have to hope for the best!
