If the chosen creature has {X} in its mana cost, X is considered to be 0.

If Protean Raider somehow enters the battlefield at the same time as another creature, Protean Raider can't become a copy of that creature. You may choose only a creature that's already on the battlefield.

If the chosen creature is copying something else (for example, if the chosen creature is another Protean Raider), then Protean Raider enters the battlefield as whatever the chosen creature copied.

Any enters-the-battlefield abilities of the copied creature will trigger when Protean Raider enters the battlefield. Any "as [this creature] enters the battlefield" or "[this creature] enters the battlefield with" abilities of the chosen creature will also work.

Protean Raider copies exactly what was printed on the original creature (unless that creature is copying something else or is a token; see below). It doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or Auras attached to it, or any non-copy effects that have changed its power, toughness, types, color, or so on.

If the chosen creature is a token, Protean Raider copies the original characteristics of that token as stated by the effect that created the token. Protean Raider is not a token in this case.

Raid abilities evaluate the entire turn to see if you attacked with a creature. That creature doesn't have to still be on the battlefield. Similarly, the player, planeswalker, or battle it attacked doesn't have to still be in the game or on the battlefield.

Raid abilities care only that you attacked with a creature. It doesn't matter how many creatures you attacked with or which player, planeswalker, or battle those creatures attacked.