Chances are if you are playing badly with one champ but not another then the fchamp you're playing with just isn't for you, not every champ can be played by a person well.
Look at games you play with Sona and Soraka, look at how you're killed. What you did that led to you getting killed, where you played them and how you played them, how you built their items then look at your enemy how did they kill you, who are they playing, what their item build is and how they we're playing. There is a reason you are doing badly with these champions and until you figure out what that reason is you will not improve with them.
The toxic thing, saying "they start it not me" does not make it okay to become toxic. I know it's hard to ignore people flaming you but reacting doesn't get you anywhere other than eventually chat restricted and then banned if you continue down the same path. As somebody said you could mute them all at the start but that doesn't really help you so much, whilst pings van communicate they don't so it as well as text and you can never know if somebody is going to flame you or offer you genuine advice at the start. If they start flaming you then by all means mute them but under no circumstances should you reality to the agresaion they show you and become toxic yourself. It not only makes the game itself less fun because you're now angry, being angry makes you play worse. You make rash decisions and get yourself into stupid aulituations that you probably wouldn't get yourself in before.
Finally if you are having a real hard time in ranked then play normals to practice more, people get less angry if you don't play well in normals (though it still happens just not as much) as they do in ranked.
