Dying in 1 combo? Unlikely unless you were massively under level and he was fed as well as a lot higher. Your team needs to deal with him like they would any diving assassin and stun / snare etc. Save your E to get some distance between his Rift Walk cooldown and plant a trap at your feet where you land and auto him. If you're walking around the map on your own then that's your own fault really and a lot of champions could kill you. You need to give more information on where you're dying, how you're dying (more than 1 combo, don't exaggerate please) and how fed the Kassadin player is compared to you.
I'm not calling you a liar but I am saying I disagree with your statement that one combo killed you outright from full HP unless he was super fed and / or you were very under level.