He's faught Death too.
He was able to call on enough power to briefly stand against MU Death, actually managing to block and dispel a couple of her attacks, when she was trying to prevent Galactus from restoring himself from the brink of death, buying him enough time to do so.
And, without prep, and when completely focused with all his power on keeping the world in existance, and barely putting any attention or effort into anything else, his defensive magics bounced an attack that made Watchers run away.
And Dr. Strange held his own against an Infinity Gaunlet wielder.
Managed to immunize himself and others from it's powers.
Strange doesn't beat Eternity
Of course not, but I wouldn't go so far as to say Eternity could -blink- him out of existance as easily as that, if he's prepared.
He'd at least have to try.