Tank Soraka is the next best thing. Great support on top of tanking is just ridiculous. She just doesn't die. If the enemy doesn't kill you, they messed up. If they try to kill you they messed up.

Pros

Great harrass
Good sustain
Very hard to deal with
Extremely deadly with another ap
Extremely tanky
Does deceptive damage

Cons

You might not get many kills if your team is around, but who cares your tanking and supporting.

So when you play tank soraka, you pretty much play the same way normal soraka would play. Or what I consider normal soraka play.

You apply constant pressure on to the enemy lane. AAing them when they get close, and when you have your starfall(Q) use it when there in range. Always infusing your partner/self so you never run out of mana. Try not to take too many minion kills if you can avoid it. Allow the carry or anyone else take them. Trust me you won't need to keep a high cs.

Another key thing in playing any type of soraka is knowing when to heal. Since the 'nerf' her heal cd is huge. So knowing when and when not to plays an important role in the soraka game.

In general you should heal if

You can afford the mana
Your partner needs it
If being attacked

If anyone is being attack say you or a partner put a heal out instantly on you or them. Remeber only heal when you absolutely need it, or when mana is plentiful and people don't need the plus armor it gives.

If they have a jungler just we aware whats going on. It all really depends on your laning partner if you have one. If you have a solid ap/ad champ with you, don't be afraid to push. The enemy should have their hp's pretty low so if a jungler does engage it shouldn't be a problem. You should have the advantage in k/d if they do fight.

Done right you should have the tower down quite early in the game. Once that happens you can go help push other lanes, and just let your partner farm top if they like.

Soraka is a great tank. So if the enemy is low or you got the items, dont be afraid of those measly towers. They are nothing. Team fights just keep spamming that Q, Heal whomever being focused. All other tanky rules apply. But if you got the items you can play pretty reckless. Try it. Its pretty funny how much you can get away with.

You can really play around with runes. In all honesty I don't even have this rune set. I run ar.pen mark quints, flat armor yellows, and cdr per level blues. Because I just don't give a ****. But if you want the best all around tanky then go with the one recommended there. Really you can put whatever runes you want on. It really doesn't matter all that much.

With masteries I run standard 9/21/0. No dodge because it such a low percentage and you really don't need it. You can probably go for 0/21/9 with points into perseverance, hands, mind and meditation for more mana/hp regen.

Why not max heal early? You wont need it. But if you feel like being even more supportive you can drop infuse(E) to get heal. Just be sure you have mana to spam starfall(Q) constantly.

Now for summoner spells I use teleport/exhaust. Why tele exhaust? Well if you see a teammate in need teleport to them. Save there life. Or jumping into a team fight and causing havoc. Remember starfall does exponential damage. Exhaust is also for saving, or just catching people from running.

Why not use any escaping summoner spells? You shouldn't be in a situation where you need summoners to escape. You can usually tank you way out if you need to run.

So what I have is pretty much core. Your pretty much unstoppable. But always check team comp just in case you need to change out other items for more magic resist instead of armor. You absolutely need warmogs though.

After those items you can pretty much go whatever you want. More tanky if you like, damage, spell vamp. I actually never gotten to the point where I'd go beyond that build, games always ended.

Be sure to pick of philosophers and heart of gold as soon as possible.

Just try it. You'd be surprise in how well it goes.