Well, I dont want to say you're playing wrong, cause if it works it works, but you don't need every element covered necessarily.
I'd recommend using 1 Strong DPS card, 1 Debuff/AoE type preferably of a different element than your main just in case, (Like Bahamut you mentioned, a Break Resist down, or something), and 2 Support cards (Cindy+KoTR are a superb combo). Or you can simply single Element your main deck and a different for your sub deck to maximum power. Basically try to make specialty decks (You have 9 deck slots + 9 sub decks), instead of one Jack-of-all-trades deck.
You could use a Jack of all Trades deck if you really want too, and if you do, i'd recommend if you go that way 1 Power card (Strong attack), 1 AoE Resistent breaker (BReak, or Crit resist), 1 Utility Debuffer (Unguard, etc), and Cindy, and bring a Ultimate Boon friend.