Frankly, you can look at past fascist regimes for the only workable answers.
https://en.wikipedia.org/wiki/List_of_fascist_movements
Italy/Germany/Japan in WWII; Ousted through military force.
There's a bunch of Axis-related fascist regimes I'm going to skip over, since it all boils down to the above.
Portugal from '33-'74; military coup overthrew the government, leading to the installation of the current governmental model once things had been quieted down
Spain from '36-'75 under Franco; Slow decline of control over decades of brutality, followed by Franco restoring the monarchy as he died; Spain's fascist regime only ended because its dictator died of natural causes and did not wish to maintain his regime.
So, of the fascist regimes the world has seen, there's really
one example where it was reformed over time with the explicit cooperation of its dictator, Franco. The rest? There was
one solution. And that solution was, frankly, liberal use of violence.
That's what happens if fascists seize power. If they don't? Well, look at all the countries that have seen fascist
movements which went nowhere. Those were opposed largely through condemnation or ridicule. Seems pretty functional to me. The best way to fight fascism is to make sure it never
gains power.