It's stupid to say Shikamaru would win solely because of his shadow techniques. Neji has the Byakugan which gives him 360 degree vision. It won't be easy for Shikamaru to catch him..
Also, the shadow techniques aren't enough to completely keep your enemy at bay. Sooner or later they will get in range. Neji's no fool. He'd find a way to get in and attack Shikamaru's tenketsu. Shikamaru's intelligence is unrivaled, but I'm not sure how far he'd make it against someone as strong and as smart as Neji
Also note, Neji wasn't the only Konoha 11 member to reach Jounin without good reason. He was already the best out of the group in their Genin days. Considering he's the only one to make it to Jounin, he's pretty god damn strong by now.