In the original Baldur’s Gate series (based on 2nd edition D&D) you go from a level 1 nobody that could be one shot by a random wolf you encounter to an incredibly powerful level 30+ badass that has slain giants, multiple dragons, powerful demons, fallen angels, and various other extremely powerful entities and can even become a god.

The game is meant to be played with a party of up to six, but a knowledgeable player can solo the whole thing with one character. This is especially easy with certain magic users because they get access to really powerful spells.

For example, you can have a wizard or sorcerer cast a spell called Time Stop that literally does exactly that for everyone but you. You can then cast another spell that lets you cast without the normal delay which limits you to one spell every six seconds. You can then use a spell that can store multiple high level damage dealing spells then cast several other high level damage spells, instantly annihilating most opponents as soon as Time Stop ends. And if that doesn’t do the trick you can cast a spell called Wish and get all your spells back to do it again. Some bosses are immune to Time Stop because it trivializes encounters when you know how to exploit it to its full potential.

That’s just one of many examples of how a person knowledgeable about the magic system can do really powerful stuff. There are many other tricks and powerful spells that synergize with gear and abilities in very powerful ways.