Magnet Sphere

You only get one orb at a time. It drifts forward slowly, passes straight through enemy bodies, ricochets off walls, and while it lives it snaps off instant beams at whatever is closest. Firing again deletes the one already out, so spamming the book is the wrong instinct. Put one in play, then let it work.

Enclosed spaces are ideal. In the Dungeon halls the orb rattles between walls for its whole lifetime and shreds anything that walks in. In a wide open boss arena it sails off into nothing. Liquids do not slow it, so flooded rooms are fine. It also opens fire on Mimics that are still disguised as chests, which is a free way to spot them.

Ruthless beats Mythical here, which is unusual for a magic weapon. An orb already in flight borrows the critical rate of whatever you are currently holding, so the moment you swap to a second weapon (the entire point of the thing) the book's own critical stat stops applying. Raw damage is the only bonus that follows the orb.