Grey Knights dominated Tyranids in 5th. In 6th, the Tyranids have more than a fighting chance. In general, 6th Edition is a shooting edition, so only dakka units really need apply.
Tyranids need MOAR PSYKERS. Biomancy is where it's at for Tyranids. Also, they have awesome Flying Monstrous Creatures. That's how Tyranids win these days.
Yes, you can shoot "around" a close combat. Follow normal rules for determining whether target unit has cover (i.e., intervening models).