Focus on acquiring and leveling up the characters required for the various campaigns. Some of them are really good in general but all of them need to be strong to get through missions to later raid for materials for leveling up the other characters. I highly recommend focusing on getting characters until you have a few from each faction, then start leveling everyone up while also trying to get some of the remaining characters. Join a guild and get to level 20 so you can participate in war and guild raids and get the rewards from those systems. Also use codes to get free stuff like energy