An "attacking creature" is one that has been declared as an attacker or put onto the battlefield attacking during this combat. Unless that creature leaves combat, it continues to be an attacking creature through the end of combat step, even if the player it was attacking has left the game or the planeswalker it was attacking has left combat.

If you cast Entrapment Maneuver after combat damage has been dealt, only creatures that survived combat can be sacrificed.

Entrapment Maneuver doesn't target the creature to be sacrificed. The target player chooses one as it resolves. No player may take any other actions between the target player choosing the creature and you creating Soldier tokens.

To determine how many Soldier tokens are created, use the toughness of the sacrificed creature as it last existed on the battlefield.