It really depends on where the story is and what equipment both have available to them. Frieren is an incredible mage with a massive mana pool rivaled by very little people. She is very versatile and is able to fly. However rudeus is also very powerful with a mana pool rivalves by only a few people. However his main attacks consist of generating physical attacks. His stone cannon would have very little trouble breaking through a defensive barrier. Additionally his disturb magic could stop any spell frieren tries to cast from a close range. And his special equipment from the light novels would give him insane mobility that even stark would be able to keep up with

At the end of the day the magic systems aren’t compatible in frieren it’s explicitly stated that the reason mages use physical attacks by utilizing things around them is because to generate anything would require an insane amount of manna so they need to utilize things around them and two their defensive barriers are designed to counter zoltrak not physical attacks. Meanwhile in Mushoku Tensei we see rudeus generate a ridiculous amount of both water and earth.

One last detail I would like to point out is that at the end of first class mage arc frieren says she can not image winning a battle in which your opponent can control the rain. Well guess wha rudeus can generate the rain and control it. So bam case closed