And this is the problem with the base of gamers here - no imagination and no concept of variety. The archdruid isn't an actual archdruid - she was second in command. The actual archdruid, when you find him, is weakened and fighting a mass of enemies that overwhelm him. The mindflayer you kill is near death - you are not fighting them at full strength. I never encountered the red dragon, but there is probably mechanics making it sensible. You look in the monster manual and you get what the average enemy is like - you can face much stronger goblins with 18 level stats and such if a DM threw them your way or you could have a feeble dragon to provide an easier time. People seem to have a difficulty comprehending that not all of one type of a creature are the exact same thing. I have to ask if their first encounter with a black person was Michael Jordan playing for the Chicago Bulls, decide that all black people can dunk with ease, and they come across Joseph Anthony Cox (the little person from Bad Santa) and assume he must be able to dunk as well because he too is black. Different members of the same group have various abilities and the ability to make strong or weak versions of a group is just decent storytelling. You don't kill the mindflayer trying to get passerbys to dig it out and get thousands of XP for it - you get what is comparable to the task at hand for killing a near defenseless mindflayer on the verge of death.
