The activated ability can be activated only if Bloodsoaked Champion is in your graveyard. Notably, if it attacks and then dies later in the turn, you can use its ability to return it to the battlefield, as its attack satisfies its own activation instruction.

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.