"Como meterse en una competencia de sangrado con un banco de sangre."

Simplified Meaning:

Trying to compete with someone who has an endless supply of something is pointless. For example, imagine trying to give more blood than a blood bank. A blood bank collects blood from many people and has a big stockpile, so no matter how much blood one individual donates, it will never be more than what the blood bank has. This saying means it's a bad idea to challenge or go up against someone or something that has an overwhelming advantage or resources in a particular area. Instead, use your energy on something where you have a fair chance. For instance, if your friend is an expert in painting and you are new to it, competing with them directly in painting might not be the best choice. Instead, focus on your own strengths, perhaps something you’re good at like writing or sports. This way, you avoid frustration and make the most of your unique abilities.