All of the Big Battles can be run solo, so worth doing that first to get a rough idea of how they work.
Worth noting they can all be done with 2 players also, which is a fun option.
The 6 man Big Battles have the same basic structure, but have slightly different challenges, and you get different quests on different runs. It is impossible to generalise re tactics as they vary a lot. Join a group ... bumble about ... and slowly get the hang of it.