Be a tall order
Definition 1 of 1
Pro Tip 1/3Metaphorically High
Imagine the 'order' as something physically high, indicating the difficulty in reaching or achieving it. 
Winning the championship against professional teams would be a tall order for our amateur club.
Pro Tip 2/3Not Impossible
The phrase suggests a daunting task but doesn't imply that it's impossible, just very challenging. 
Landing a spacecraft on Mars is a tall order, but with enough research, it's feasible.
Pro Tip 3/3Game Context
Often used in sports or games to express that winning or succeeding against the odds is tough. 
Coming back from a 3-0 deficit in the finals would be a tall order.