Speed of light and mantra are still irrelevant in such a fight. One of them could attack the other one in their sleep and nothing would happen. Lightning is not vulnerable to light, and light is not vulnerable to lightning.
Edit: Lol, I can't really counter-argue a point made via rep, especially over manga content, since I can neither post here nor in PM. However, a fight between two paramecia types is a little more up in the air than two logia types. Plus, don't forget that *spoiler stuff etc etc*
Basic rule of thumb: With logia, it's considered immune unless proven otherwise. With paramecia, it's considered vulnerable unless proven otherwise.