@Philipp gave a great answer. Another concept to take into consideration is the idea of average threat level. It is perfectly acceptable for certain foes to have stats dramatically greater than what your curve predicts – so long as their other stats are lowered in proportion to the increase. For instance take pixies armed with envenomed needles. They are extremely fast and small, represented by a high dexterity stat (or speed if that is what you wish to call it) and a corresponding raise in evasion. However, this means players will likely miss them more often, so you should lower their hit-points in proportion to the change. So if the player is only supposed to be required to attack a foe x times to kill it, and the pixy has a 50% miss chance, you can give it approximately half the health of your average foe. Likewise giving the pixy an attack that grants a status effect can make up for it having a low attack power.

This allows you to make your enemies more unique and will make the game less boring. Rather than the goblin, the golem, and the giant all being the same except for the number of wounds they can take, you can push certain features of theirs in one direction and reduce the others accordingly to keep it within your target challenge level.

@Philipp gave a great answer. Another concept to take into consideration is the idea of average threat level. It is perfectly acceptable for certain foes to have stats dramatically greater than what your curve predicts – so long as their other stats are lowered in proportion to the increase. For instance take pixies armed with envenomed needles. They are extremely fast and small, represented by a high dexterity stat (or speed if that is what you wish to call it) and a corresponding raise in evasion. However, this means players will likely miss them more often, so you should lower their hit-points in proportion to the change. So if the player is only supposed to be required to attack a foe x times to kill it, and the pixy has a 50% miss chance, you can give it approximately half the health of your average foe. Likewise giving the pixy an attack that grants a status effect can make up for it having a low attack power.

This allows you to make your enemies more unique and will make the game less boring. Rather than the goblin, the golem, and the giant all being the same except for the number of wounds they can take, you can push certain features of theirs in one direction and reduce the others accordingly to keep it within your target challenge level.

Also, while it is usually preferable to keep the ATR relatively even (bosses should always have something neat that surprises even the best prepared players), it never hurts to shake things up a bit. Take Diablo 3’s treasure goblins for example. Weak in every way, yet drops amazing loot. It only contributes to the dungeon’s difficulty by convincing otherwise intelligent players that it’s a good idea to go running headfirst into unknown territory because “otherwise the little bugger is going to get away with the loot!” Yet no one complains about them, because it is an unexpected and refreshing challenge that brings its own reward.

Another example I love is the earth dragon from “Golden Sun: The Lost Age.” Its stats are pretty high, it regenerates an insane amount every turn and it can’t actually be killed in a fair fight (I tried for kicks once). This is dealt with when the player figures out that there is an entire area inside that mountain dedicated to weakening the dragon and stripping it of its regeneration. Sometimes it is great idea to make foes absurdly overpowered (by the numbers) and then introduce other variables like volatile terrain or legendary, boss-specific weapons. The key thing I have learned about this is to keep it fun.