There are only a few characters who Cell might not be able to kill, and even then...They would not be able to kill him.
Madara, due to phasing powa. But Madara cannot kill Cell, and would prolly die if he made the planet pop.
Kizaru, Cell can't hit light. But Kizaru cannot kill Cell in any way.
Eneru: The only character Cell cannot kill, even by planet popping, but Eneru can't kill Cell, so Eneru should prolly just run away...Lol.