Game mechanics dictate how challenging an enemy should be. A game mechanic is any action that takes place within a game. The fewer mechanics in a game, the less information the player has to memorize, and the easier it is for designers to balance gameplay.
Advertising games as being something that everyone can play initially sounds like a good idea for making money, but trying to appease a broad audience can create a plethora of mechanic options. This leads to overcomplicated situations that confuse or frustrate players. With a limited number of ways to defeat an opponent, the enemies must be planned accordingly, usually limiting unique traits to one per enemy. Traits can be defined by the tactics or behavior the enemy pursues to gain victory. Examples of this are enemies that run directly toward player, enemies that take cover, enemies that show patterns of avoidance, and those that play a support role.
When comparing this against the hierarchy of challenge, opponents can have multiple tactics to increase the challenge, but that is not the only way. Once a trait is chosen, the next choice is form of attack. The most common forms are close, ranged, and support. These main forms tend to have rock-paper-scissor rules associated with them -- in other words, range type beats close, close beats support, and support beats range. This allows for a secondary level of tactics without adding more mechanics.
Another alternative to consider is how the enemy spawns or enters a scene. This can have a major impact on the effectiveness of the enemy and its ability to keep the player entertained. Weaker enemies that over time become repetitive can change spawning patterns to vary game play and even increase the challenge the enemy presents.
Bastion, a game with very few mechanics, starts with the player fighting weak one-hit-kill critters. As the game progresses, these monsters are launched out of cannons. The enemies fly through the air, crash into the player, then continue to perform their usual behaviors. At other points, a large enemy force spawns in the form of falling boxes. The player can easily shatter the boxes, destroying the creature within before it has a chance to break free and then become much harder to kill.
Call of Duty: Black Ops 2 presents a good example of what not to do in its Zombie Mode. Players learn the first rules of the game in a small barricaded room. Zombies spawn and then have to break through the barricades to get to the players. This establishes a rule that zombies spawn on the other side of barricades, which players cannot get past. Later, the zombies randomly pop up from the ground, even when barricades are present. In essense, the game promotes teamwork by encouraging players to watch barricades, but then punishes that effort inconsistently.