Congratulations on beating her! I'm delighted I could help. The game does not explain how to use Boost at all; I have no idea how anyone knows, frankly.
A tip for getting strong magic later in the game: once you have this game's airship equivalent, fly it over to the westernmost island on the map. This is the Island Closest to Hell; you can check your location in the menu to make sure you're in the right place. Make sure you've got encounters turned off (the correct way to play this game, frankly!), because the enemies here are powerful, and then run around the island, repeatedly hitting the 'interact' button. The island is covered with draw points for powerful magic: Flare, Meteor, Aura, Full-Life etc. They'll replenish over time, so you can come back later if you want more.
There's also an Island Closest to Heaven on the eastern edge of the map, similarly covered with draw points, but that's a little harder to distinguish at a glance, so I'd suggest looking up its exact location.
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A tip for getting strong magic later in the game: once you have this game's airship equivalent, fly it over to the westernmost island on the map. This is the Island Closest to Hell; you can check your location in the menu to make sure you're in the right place. Make sure you've got encounters turned off (the correct way to play this game, frankly!), because the enemies here are powerful, and then run around the island, repeatedly hitting the 'interact' button. The island is covered with draw points for powerful magic: Flare, Meteor, Aura, Full-Life etc. They'll replenish over time, so you can come back later if you want more.
There's also an Island Closest to Heaven on the eastern edge of the map, similarly covered with draw points, but that's a little harder to distinguish at a glance, so I'd suggest looking up its exact location.