Meaning: a big task to finish
Rewrite 1: a deep leak to bail
Rewrite 2: a long hill to climb
Comment: You want to preserve the sense of difficulty, yet keep it in the same simple terms as the original, depending on your audience. For instance, if you're writing for mathematicians you might prefer something more like "a long number to crunch" or for surgeons something more like "a deep laceration to stitch."
Writing Tip: It isn't always necessary to be clever. Often, saying something clear and straightforward is far more effective. In these cases, it's best to invoke the KISS principle: Keep It Short and Simple.