While the second and third abilities of Champions from Beyond have "Party" in their flavor words, neither of them refer to a player's party (a mechanic from previous releases that also uses the term "full party").

The second and third abilities of Champions from Beyond don't care what happens to those creatures after you attack with them. Once the second ability triggers, you'll scry 2 and then draw a card when it resolves even if one or more of those creatures left the battlefield or were otherwise removed from combat. Similarly, once the third ability triggers, those creatures will get +4/+4 when it resolves, even if one or more of those creatures left the battlefield. Any of those creatures that were removed from combat but are still on the battlefield will still get +4/+4.