Since all of your Pokemon must share a type, monotype usually revolves around covering your Pokemons' shared weaknesses, and taking advantage of opponents who don't. This answer should help you get better at doing the above.
Pidgeot: Replace this with Lopunny, which is extremely helpful against rock, steel, and ghost Pokemon.
Lopunny @ Lopunnite
Ability: Limber
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
-Fake Out
-High Jump Kick
-Return
-Ice Punch
Its ability and extremely high physical attack and speed makes it arguably the best offensive normal type in the metagame. Due to its ability, it can hit anything for at least normal effectiveness using only its STAB attacks. Fake out prevents opponents from outspeeding it on the turn it mega evolves. Ice punch is used against common ground type tanks, such as Gliscor, Landorus, and Garchomp.
Porygon2: Replace hidden power with thunderbolt for more damage and better coverage with ice beam. Give it 252 HP/100 Def/156 SpD Bold for better special defense.
Heliolisk: Replace grass knot with surf for more damage.
Bibarel: If you want a good special wall, use Chansey.
Chansey @ Eviolite
Ability: Natural Cure
EVs: 252 HP / 252 Def / 4 SpD
Bold Nature
-Thunder Wave
-Softboiled
-Seismic Toss
-Heal Bell
Thunder wave prevents most sweepers from sweeping. Softboiled allows Chansey to heal itself. Since Chansey's attack stats are so bad, seismic toss is generally regarded as the best way to damage Pokemon that know taunt or are immune to toxic. Heal bell supports the rest of the team.
Wigglytuff: When I suggested taking out Pidgeot, I didn't forget to consider your fighting weakness. Staraptor is a good flying Pokemon that doesn't need to mega evolve.
Staraptor @ Choice Scarf
Ability: Reckless
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
-Brave Bird
-Double-Edge
-Close Combat
-U-Turn
Brave bird and double-edge are STAB attacks. Close combat covers ice, rock, and steel, whose Pokemon may resist Staraptor's STAB attacks and/or hit Staraptor for super effectiveness with their own STAB attacks. U-turn is used if you don't feel like getting choice locked. It can also save the need for prediction, as you see what Pokemon the opponent switches to before you decide to switch. You may want to replace reckless with intimidate. If you do so, Staraptor won't be as powerful, but you can spam u-turn against physical attackers to lower their physical attack.
Zangoose: If you want a better all-out attacker, use Diggersby.
Diggersby @ Choice Scarf
Ability: Huge Power
EVs: 4 HP / 252 Atk / 252 Spe
Jolly Nature
-Return
-Earthquake
-U-Turn
-Wild Charge
Its huge power and immunity to paralysis makes it a good wallbreaker. When holding a choice scarf, it becomes a sweeper and a revenge killer. Return and earthquake are STAB attacks. U-turn covers grass, prevents Diggersby from becoming choice locked, and saves the need for prediction by allowing you to decide what to switch to after the opponent switches. Wild charge covers water and certain flying Pokemon that take advantage of their ground immunity.
