If I cast a spell that targets/chooses an opponent's unit and my opponent removes the card before resolution, does my spell fizzle or do as much as possible given the text of the card?

In Riftbound, spells do not "fizzle" in the traditional sense of being countered or failing entirely just because a target becomes invalid. Instead, they resolve as much as they can.

According to Rule 356.3.e.1, "The spell resolves even if some or all of its targets are illegal."

Here is how the interaction works when a target is removed (e.g., moved to a base) before resolution:

1. Targeting is finalized at cast: You must choose a legal target when you play the spell. You cannot choose a new target if the original one becomes illegal.
2. Legality check at resolution: When the spell resolves, it checks if the target is still legal (e.g., still on the battlefield if the spell requires it).
3. Mistargeting: If the target is no longer legal, the specific instruction targeting that unit is ignored (it "mistargets").
4. Resolution of remaining effects: The spell continues to resolve any other instructions it may have. If the spell has multiple effects (e.g., "Deal 4 damage to a unit at a battlefield. Draw 1 card."), the damage portion will fail to execute, but the draw card portion will still happen.

Key Rules:

• Rule 356.3.e.5: "If any of the spell's targets are no longer legal, those targets are unaffected by the spell as it resolves."
• Rule 356.3.e.10: "It is possible for none of a spell's instructions to be executed as it resolves... In this case, the spell has no effect but is still considered played."

Summary: Your spell will not fizzle; it will resolve, and any part of the card text that does not rely on the now-invalid target will still occur.