In explorers cabin my hero’s are sometimes 10 levels above the other player so why do I get slaughtered?

Although your team may have a higher level, it won't insure victory. Other factors are used to create results. Such as: gear level, skill level (skills are different in expedition vs. exploration), hero's star level tier, formation, etc. I find its beneficial to keep 2 marksmen and put them behind my lancers and infantry. It gives them a better chance of inflicting more damage before they start to take a hit. That is just one example. Hope this helps!