Karna should be able to take this. Ishtar's only shot at winning is potentially dropping the concept of venus in Karna. (Which would be negated by 90%, so I doubt it would one shot him.) However Vavasi Shakti broke Achilles's shield which is the concept of a planet itself, Karna should be able to stop it. Then he'd go on to using Brahamastra. If Karna ever got the chance to use Vavasi Shakti offensevely besides stopping Ishtar's noble phantasm, it would inevitably one shot her due to her status as Divine Spirit.
Coupled with his Prana Burst (Flames), Karna should take with with medium difficulty. Not to mention Lancers are naturally strong against Archers because they lack exceptional close combat skills.