WARNING: The following contains spoilers for Episode 1 of I've Been Killing Slimes for 300 Years and Maxed Out My Level, now streaming on Crunchyroll.
The world of isekai anime is an intense form of escapism, where the protagonist can break free of the tedious and familiar grind of real-life and run free in a fantasy realm far, far away. This can be a lot of fun, but too often, such series end up as power fantasies for the (typically male) lead, which gets old fast.
Not every isekai series takes this route, not even all the action-oriented ones, but plenty do. In such series', the escapism is taken a notch too far, and the series quickly stalls out when the main character becomes all but unbeatable. He or she (usually he) commands immense magic, a circle of utterly loyal friends and basically everything they could ever want. Such a character has reached the end goal, which means there's nowhere left to go. Thankfully, series' like I've Been Killing Slimes freshen things up simply by changing the main priority. Why become an invincible hero, when you can stop and smell the roses?
Some parody or comedy series have commented on the issue of overpowered protagonists, like the ever-popular One-Punch Man. In it, Saitama has become nigh-invincible but faces borderline depression and boredom as his true enemies. In other series, the invincible lead, like Reincarnated as a Slime's Rimuru Tempest, seeks to improve the lives of everyone else instead. Not only is that noble, but it means that the character's combat power doesn't mean as much, since their problems revolve around fixing problems and relationships between and for other people.
Other isekai series dodge the "invincible protagonist" issue by giving them nonviolent goals that cannot be accomplished with violence or strength, like seeking a purpose in life or falling in love. That's what I've Been Killing Slimes for 300 Years and Maxed Out My Level does, and where Azusa, its invincible witch heroine, comes in.
True to the series' title, Azusa spent three entire centuries killing slime monsters to grind XP and gold, and her stats are all maxed out. Because of this, no battle is even remotely challenging to her and, in a combat-oriented series, this would be a real problem. However, Azusa's goals are not based on fighting or glory -- she only wants to chill out.
Azusa is incredibly strong, but that doesn't matter, since her goals are simple and pacifistic. Her magical strength is incidental, meaning the true challenge is to see whether she can maintain a quiet, happy life amid constant interference from warriors wanting to test their combat skills against her. This leads to a charmingly silly and heartwarming story where Azusa rolls up her sleeves and gets to work making sure she (ironically) doesn't have to work ever again. She will paradoxically push herself hard to take things easy and is ready to instill this lesson in her new friends, such as Laika the dragon girl.
What's the point in power, fame and wealth if one can't enjoy life's little wonders along the way? Ultimate power is boring on its own. Thankfully, Azusa knows that happiness is something that comes from within a person's heart, rather than atop a pile of defeated enemies. It's been argued that happiness is a decision, and Azusa has humbly chosen wisely.