Just a few suggestions from the top of my head.

-Score a goal from x distance.

-Block x amount of goals

-Center a goal for a teammate to score.

-Equip a new (rare) bow, block, and character

-Win x amount of games in x game mode.

-Hit a ball x amount of times in a row without missing.

-Score within the first 3 seconds after the match starts.

-Score a goal by hitting it with a block.

-Keep the ball from touching the ground for x amount of seconds.

-Make an other player go boom by hitting them with the ball/ running into them really fast.

-High five 5 players