Do you even realize that Sona's W is not a damaging spell? That's her heal. Also, 150 MR is nothing. Again also, if we're talking late game, you have to take into account Lich Bane and Luden's Echo procs that deal a good part of her damage for her.
What you got hit with is probably; Maybe one Q, one AA, plus Q proc, Powerchord proc, Lich's Bane proc, and Luden's Echo proc.
at lvl 18 fully maxed, that's 160+50%AP (Q), about 100 damage from AA, 330+28%AP from Powerchord, 75+50%AP from Lich's Bane and 100+10%AP from Luden's Echo.
Totalling at 765 base + 138% AP scaling.
Assuming she is fully built and at around 700 AP which is realistic; she'll deal 966 damage from ratios.
Total damage; 1766HP mitigated by whatever magic resist you got.
If that Sona was rocking Thunderlords, add another 250 damage.
And THAT, sir, is NOTHING compared to what a Leblanc or an Anivia can do to you.
Sona is a support, but she's also a mage. It's your bad for not building against it.