Heal the hell outtah your alli
Warding item + Boots
Against heavy AP + CC
Full Build Example against heavy AD
Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Go for flash & heal because the most time you will not be in reach for your exhaust (Or at least you should not be in reach of them). For heavy pressure pick ignite withya (Recommended for a healing enemy team. If your adc goes crazy because ignite or heal pick exhaust. Some adc's cannot play with exhaust.
PS: If your adc is Caitlyn maybe double heal would be more effective because you both are not going to be very close to the enemies.
Must-Have + Trinket
Go and Rush 45% CDR !! Must Have as well is the Talisman. Ardent Censer is a hilarious good item for Soraka because you can easily buff the whole team with AA-Speed as well as health & MR/Armor.
For you trinket you can use whatever you fancy to or what is needed. Situational.
Soraka's Silence Spell is a very effective weapon against enemy combos. Try to look ahead what spell your enemy tries to cast soon and destroy it with your E.
Casting spells like Fiddle's W or R can be stopped as like Teleport. Use that to your advantage.
Sorakas do not need any AP to be OP. With this cheap build you are going to be pretty solid and strong as well.
GOOD LUCK SUMMONER.