When designing Enemies, they can generally be separated into actual Enemies and Bosses. Some of you may be tempted to work on bosses since their fights stand out more, so let's look at Bosses first.
Now, there are different opinions on what makes a good or bad Boss. A basic boss would generally take more hits and is able to outpace your normal attack pattern; Bowser and Robotnik are two reoccurring bosses where jumps and spindashes are not as effective and other means of attacks takes time to whittle down their health. You may also notice that both villains set the standards for the rest of the following bosses, where Bowser adds more attacks while Robotnik goes for different patterns. For Bowser, even if he throws more hammers and fireballs, players at this point would notice a small but safe pattern to get past him and drop the bridge.
What you may have noticed is that many boss fights have a sort of pattern to them. While I'm tempted to follow World of Warcraft's boss mechanics, I also take a look at boss fights from other genres like platformers and shooters. Let's take Graveward from looter-shooter Borderlands 3 for example. The first type of pattern is his Attack Patterns. He will usually open with a Slam that tilts the ground and causes a wave of boulders to roll down, then he will generally Punch or Sweep the battlefield, and if not dealt with quickly, he also Summons Mobs. However, Graveward is a Damage Sponge that was patched to not take much Damage Over Time, which is where the Weakness Pattern can be exploited; other than his chest, Graveward leaves himself open to big damage after a Punch and if his HP is down to a certain point. Of course, these patterns also affect a player's reaction, from making sure they don't slip down the slope to dealing with mobs and shooting the critical points.