Don't bother with defenses.
As you said Pikachu is very frail, and so you shouldn't bother training him in defense, instead, train him in attack or special attack, depending on what you're running. You can also run mixed sets, which can work well because it doesn't really matter if the nature lowers defense. Although honestly Pikachu is terrible in competitive, even with the Light Ball, but he does OK in lower tiers I guess, if you play Smogon. So the nature I reccomend is Timid for special Pikachu, and Jolly for physical Pikachu and Hasty for mixed Pikachu,because his speed is good, but not good enough. And yes, the Light Ball does matter, because it boosts his attack and special attack by 50%. In fact Light Ball is probably the only thing that makes him remotely good.
Here is a list on natures if it helps.
Hope I helped!