So I think I've discovered the secret to completely destroying Dominion.
The secret to success:
Roll with AP Quints, AP Seals, AP the others ones, MagPen Marks. 21/9/0 Masteries. Take Heal and Promote. Start with Boots of Cooldown Reduction and a few pots. Go bot.
Congratulations: you are now faster than 99% of the players who start bot and do not take level two boots. Take two levels of Starcall, one of Infuse. Max Starcall, then Infuse, then Heal. Your AP runes give you a little bit of early game oomph that will be multiplied at least a little when you finish your Deathcap. Hoard the bushes on bottom: if you can get a silence off on your opponent, you now have an absolutely free window to get a little closer, smash them with starcall, and back off before they can retaliate. Play smartly to control the health pack and to back off when you need to let your pots or heal do a little magic if you do take retaliation (usually in the case of AD). Cast Starcall basically on cooldown, and you will absolutely demolish the creep waves as well as stacking up a Magic Resist debuff every time your opponent gets close enough to be hit by it. If you're confident that their team isn't lurking above you, Promote and push the minute your opponent realizes "holy shit, I cannot beat Soraka" and starts to retreat. If you ever suspect the tiniest bit of a gank, back the fuck off. Soraka will wreck almost anyone 1v1, but 2v1 is a losing proposition.
Ideally, stay in lane until you have 1600 gold. Get a Needlessly Large Rod, and now punish whichever idiot stayed in lane with you. Or, like my last game, when Heimerdinger leaves for top after dying, wreck Wukong when he comes down. When Wukong heads back top after his shameful defeat, destroy Ahri. Say in All Chat "I'm making so many new friends!" End game 10/2/15. In any case, if nobody has cottoned on to the fact that you are an absolute monster, go ahead and finish a Deathcap first. If you are having close calls at the end of fights (or just have bad timing at facerolling HEAL + ULTIMATE HEAL + SUMMONER HEAL all in a row to holy shit she just went from 10% HP to 100% and I am still at 20%), build defense against whoever you are against. Frozen Heart for AD, Not Banshee's Veil for AP. By the end of a long game, you should probably have Deathcap, CDR Boots, Rylai's, Frozen Heart, and Not Banshee's Veil, because make no mistake, your biggest weakness is going to be being burst down hard and fast. Stick to 1v1s, or else revert to "surprisingly high damage support" Soraka and use your team as meat shields once people start trying to put the hurt on you. As always on Dominion, don't fight a losing battle if it won't slow the enemy down. If you can successfully stall for several seconds BEFORE going down (and as Soraka, standing on your point and managing your three heals while starcalling the entire world you can stall for a pretty good while if you're not being CC locked), then sure, hang out.
BOT CHAMPIONS I HAVE ABSOLUTELY WRECKED: Ryze, Tryndamere, Heimerdinger, Alistar, Kassadin, Katarina, Yorick.
MAY BE TROUBLE: Sivir and Ahri have both given me trouble. With Sivir, you have to bait her spell shield, break it with Starcall, and then Silence. Or silence before she can shield. The Ahri I had trouble with built a little beefier (boots, Prospector's, looked like some HP runes), and if she hits with her charm, you can be hurt. In general, auto attackers will be a rougher proposition since they can still retaliate decently while silenced, but if you can properly bait them into Silence / Starcall traps, the true secret to Soraka's success is that she will win every single battle of attrition that she can control. Just don't let them burst you down. Play it safe at ranged if you start to smell trouble, because no matter how many creeps they can build into a wave, you're only two or three starcalls away from wiping it out.
When four descend on your point, know as you die that you are the one who has brought them to this, and weep quietly in joy.