How do I deal with Leblace and Malzahar?

I've been playing Karthus for while now, and have had decent success with him, winning, or atleast not dying while get I a few kills in other lanes with my ult. The likes of fizz, yasuo, kass and other assassins/mages have never really given such trouble I do as malzahar and leblance. Some thing I've tried was to out push Malzahar, which actually worked untill he got lvl 6 and, well, kill me. I cant farm under tower since he pokes me to oblivion, and his ult cd feels like its from an assassin, he always has it. Later on in the game, im already behind in most aspects, and he still has good team fight ability's. With leblance, I really haven't found a way to deal with her pre or post lvl 6, she bursts me, tower dives me while only receiving 1 turret shot since she goes back immediately, her E is pretty easy to dodge when she isnt i your face, which she usually is when she throws it, and leter in the game, she burst's me like im an adc. The builds I usually do Is: , ,, , , Another build I've been trying recently, and seems to work is: ,, , , , or , since most give health stats, making me a little "tanky" I also have trouble with them and zed when I play Orianna