The campains in the past our characters have passed easily, no one ever dies because they would just complain and everyone is getting attached to their character. All of us but one changed our characters for this next campain, the seneschal learning from his mistakes before made it super hard. Of course, the stupid people died quickly and i'm glad that they did, but our one strong character didn't show up for a scenario and we all got wasted. Not because we made stupid mistakes but because it was just too hard for us little guys. What should we do about the different characters?

Well, YMMV on this but I have long ago discarded the idea of ever playing a session where key people are missing. And IMO if the game is being run right, everyone is key, and therefor if not everyone can show up, the game doesn't happen. Break out Axis & Allies or start another campaign of something prep light and not too intense.

If someone regularly has trouble showing up, they just plain don't get to play any more.

Draconian? Yes. But IMO, intense powerful roleplaying that is more meaningful than a mere dungeon crawl requires a level of committment from everyone that makes the presence of everyone required.

So my answer to "what should we do" would be "you should have played something else that night".