The Elite Four, as the name might suggest, is comprised of four Elite Trainers, some of the best found in the entire Kanto Region. In order to become the Kanto Region Champion, you must first defeat all four Elite Four Members in battle, consecutively no less. The second member of the Elite Four is none other than the man known as Bruno.
Brunco specializes in Fighting Type Pokemon, but he will also carry one Rock Type Pokemon into battle as well. All of his Pokemon will be at Level 52, except for his Machamp that will be at Level 53. In all, he will battle using Onix, Hitmonlee, Hitmonchan, Poliwrath and Machamp. Since most of his Pokemon are Fighting Types, Flying Type Pokemon do especially well against him. Fighting Types hate being hit with Flying Type Attacks, and their own Fighting Type Attacks don't do much damage against them. On top of having at least one Flying Type on your Pokemon Team when going up against Bruno, you should also have a Grass Type or Water Type in order to deal with his Onix since most Flying Type Attacks are useless against it.
Pidgeot is a Pokemon most trainers will use throughout their journey of the Kanto Region, since Pidgey is one of the most common Pokemon found in the Region, and it does great against the Fighting Types Bruno owns. Pidgeot won't fair that great against Onix, but you could have another Pokemon on your team, such as a Grass Type or Water Type, in order to defeat it.