A couple comments:

Nexus is easier than UK, in my opinion. You may want to focus on it.

You mention the "1st Boss" of Nexus, but there really isn't one. It's non linear and you can do any of 3 bosses "first". Which one are you struggling with?

I have run all instances up to Halls of Lightning with my 4xHunters+Druid team. Some of them have taken a LOT of time to learn, however. Don't give up. Like others mentioned, read strategies for normal groups and adapt to your multibox mixed group as needed. There are certainly very few "tank and spank" bosses in LK, but I think that fact favors mixed groups more than the standard 4x/5xShaman setup.