Pretty much any deck that relies only on shields to survive only works on a few decks. However, defense is always (or maybe just usually?) the overall strategy against FG's. Rainbows are successful because so many of their cards are defensive cards. Take out bone wall, otyugh, fallen druid, quintessence, SoG's or bonds, eternity or pulverizer, steal, RoF, and any other extra defensive cards and your chance of beating any FG's is rather low. Unless you know which FG you're going to fight next, then you can use any deck that is based on defeating that particular FG. The only really offensive cards in the standard rainbow are FFQ's, graveyard, and fallen druid/elf is both offensive and defensive. I'm pretty sure I included everything offensive/defensive in standard rainbows...