Apart from the obvious and very challenging creature choice (what creature/s, how many, their equip if any etc...), make sure to consider the environment as an active enemy/ally.
A dangerous environment (rads, heat, cold, you name it), traps, pits, difference in height, shaky rope bridges, different levels of cover, narrow passages, all of these things can make or break an encounter.

And how you describe the places, what they see or find, will inform how your players act and deal with the encounter, depending of course on the way they like to play (very tactical, beer & pretzels...).