^ What he said.
Rain of Fire is Crazy strong, and relatively cheap to get rolling. You at bare minimum need a crafted/rare staff with +11-12 druid skills and good spell damage roll (100+ is pretty common), and -200 pierce (so that you always have effectively -100 pierce when a boss drops it's immunity to fire, even if it's base 99%), which is also super easy to get with bare minimum crafts. Just be sure to clear heroic areas with Traprat first if you think you're too squishy to do it as a caster alone.