This boss fight may take a while if you want to spare her as the only option you can choose to win passively is to 'Spare' her. Keep doing so and eventually, she will give up. Toriel has four different attacks. Fire will fall from the top of the box - stay around the middle ...
Included is information on how to spare enemies in case you're interested. Please do not farm enemies, as there are a limited amount of them and killing them all may cause some issues later on. Whismuns can be spared immediately, all of the time. You can also console a Whismun to ...
During the enemy turn, all enemies will attack at once, but usually at an easier rate than when that enemy is alone. As such, if facing both a Froggit and a Whismun, it may be wise to spare the Whismun to get it out of the way. All attacks should be fairly easy to avoid anyway...