If you know how to play Raka, it should not be an issue
Just like medics in every other videogame that has ever existed,** the optimal Raka strategy is to never get hit by the enemy**; the only thing that should be dropping your healthbar is your own W.
And this is very much achievable as you should not be in the front line or even the backline, you should be behind your carries forming a backline to your backline so that your enemies must dive through two layers of teammates to reach you. This good positioning will ensure that you never die and in doing so ensure your team never dies either. Soraka's win condition in a teamfight is to stay alive, since so long as she lives, no-one on her team will die.
In my most successful victories with Raka, I've gone through 40 minute games and taken less than 10k damage from the enemy team. If the enemy ADC is hitting you (and therefore applying grevious wounds), you're playing raka wrong. You should never be in range of any enemy. You need only to be in range of your teammates to heal them. Your Q also has a pretty massive range to it, and is guaranteed to hit when the E snare goes off (and even without the E snare, the mosh pit of the frontline in a teamfight rarely moves meaning you will probably hit 2-3 champs and get health).
If the enemy is hitting you, grevious wounds or not, your Q isn't going to save you. there's a lengthy travel time and the healing is usually somewhere between 70-150 hp, which is about half an auto's worth of damage. Soraka is very fragile, so much that if they are hitting you, you're proabably dead. Her Q is not a good defensive/tanking tool, her E is a good self peel tool because of the snare, but relying on her Q to keep you alive through the damage of any enemy champion is not (and never has been) viable.