For a guide check out Flamethrower's set of guides, the Fanatic one is good and a different style than I play her.
Fanatic is my favorite Hero to play. She feels like you are just recklessly flinging yourself into battle and smiting everything until you can't smite anymore, then resurecting to resume the smite.
My moto for Fanatic is: "The best offense is a strong offense, the best defense is an even stronger offense."
My advice for playing her: "Don't fear death."
Fanatic has a ton of situational cards, but almost every one of them is epic when their time comes. Two main playstyle changes are between when you are facing lots of minions and when you are facing one big enemy.
Zealous Offense is Huge in a minion heavy game. There are a lot of ways to deal the three damage to keep it out, but the first turn is always the hardest since you don't get a one-shot to do damage with. It is important to remember the many cards that can work to get you that three damage to get Zealous rolling.
Absolution being in play for your power or an ally getting you a +1 to your base power are the easiest ways. But also any card that deals damage at the start or end of your turn works well too. With Divine focus you can discard to start your turn, then let focus destroy itself and you only have 1 daamge left to make up the three recquired, same with Chastise (dealing damage to yourself is still dealing damage). Embolden on yourself works wonders, since not only do you deal yourself 2 damage at turn end (which if it is in play first is before Zealous Offense would destroy itself) but it gives you a second power to deal damage. Embolden is probably the best way for Redeemer Fanatic to keep Zealous out, since it lets her use her base powr to heal and draw while still dealing damage to keep Zealous out.
The last is Sacrosanct Martyr. An incredibly situational card, but remember damage you deal yourself counts, so if the enemies are taking -1 damage you can deal yourself 2 damage, another target 1 and 3 damage has been dealt.
Vs. one big villain Divine Focus is huge, since it can really turn out some serious damage, esp. with some +damage boosts, which also will let Undaunted reduce the upkeep cost. Unless it has big implication outside of just dealing damage resist discarding cards that give you draws. Prayer is the best card to keep, esp if you are going to keep focus around. Empty your hand and then fill it back up, losing your turn hurts, but you are doing it for 6 discards for Divine Focus. This has the great effect of burning your deck really quick, helping you find the crowning jewel of Fanatic's deck, Wrathful Retribution.
The last card I'll go on about is Sacrosanct Martyr, while not a great go-to power, it has some great uses. Remember the motto, Sacrosanct Martyr is awesome for killing those targets that your team can't quite kill. At that point think about the trade off, if that target is going to do a good amount of damage, play villain cards or destroy stuff there's a good chance that sacrificing your HP to take that effect off the board is a win for your team. Sometimes Chastise is a better option, since you can just lock it in play, where it can't deal damage or go to the trash and come back out. As an example: Hired Gun is a great target for Chastise, where the Contract is a better target for Martyr. (assuming you can't otherwise dispose of them and had both available)
The last piece of advice for Fanatic is Visionary. Fanatic and Visionary are wicked together, two goddesses of combat both with a healthy disregard for death, they work well together, shutting down and redirecting villain damage, Visionary can search Fanatics deck for Retribution, and get back ongoings (which is awesome with Fanatic) and most of all: Twist The Ether. Twist is insane on Fanatic. lower the cost of Chastise or Embolden, boost her base power to 2+2, and change Chastise or Divine Focus' start of turn cost to radiant, which of course chains awesomely with Martyr. (since martyr deals damage equal to all the radiant Fanatic has taken that turn)
If you are looking at your hand and it feels like you have epic cards and nothing to do, well that is because of the situational aspect of her cards. Don't be afraid to get cards in play before they are needed. Zealous you want out early, since it doesn't take effect for a round after you play it. Divine Focus is strong, but it is the opposite, play it whenever the villain isn't going to just heal back to full or reduce the damage to 1, because there isn't a "right" time to play it. The biggest thing is to let go of trying to play her right and just jump in with the most smiting you can.
Edit: forgot something, remember as Fanatic you have the best heal in the game, if you play to stay high HP you will never use it. Think of Divine Sacrifice, you eat damage for your team, you end up low HP, but low HP is fine or Fanatic, slap down Aegis and allow yourself to heal. Taking 16 damage using sacrifice that would otherwise hit all four heroes for 4 each is a good deal when you can get 10 of that back later. No one else gets to run out of HP to heal awesomely, don't be afraid to use it.