DragonBelow I have two questions for you.
First, are you equating fun with challenge, or are you saying you desire both? I've GM'd and played in many games with very low challenge that were a ton of fun, and some of the least fun games I've been in have been HardMode [although there have been fun Hardmode games as well of course.]
Sure, I just think that games where the PCs mop the floor with their opponents on a regular basis, and without breaking a sweat, are not fun.
Second, not every enemy needs to exploit a One Trick Pony's weakness. Perhaps 1/4th to 1/2 should suffice.
That's still too much, and there is no good explanation for it, also that means one, sometimes two times per session. Not to mention that then it becomes an arms-race between PC and DM, which makes things uglier.