The magic system is a little confusing. They say Susie's overpowered magic resulted from using unregulated magic without a spirit contract. Does that mean anyone could unleash strong magic without a spirit? But they also say even unleashing such magic is impossible. Inconceivable! These differing explanations make me think the magic class isn't fully aware of what's possible, or more likely that Susie is unique (like an isekai protag tends to be!).
I expect that Susie is about to summon something crazy. The Undine Queen, or the Great Water Dragon, or the Kraken. You know the usual suspects.