Ok, Lance has 3 Underleveled Dragonites, A Gyarados, A Charizard and an Aerodactyl. So the best things to use against the Dragonites Is an Ice type move like Blizzard or Ice Beam because Dragonite has a 4x Weakness to Ice, you can also use rock Due to You having Onix. For Gyarados you can use a Rock or Electric type (Preferably use Electric Moves on Gyarados due to 4x Weakness on Electric). For Charizard and Aerodactyl you can Use Water, Rock or Electric Type moves. You can also use Steel Type moves against Aerodactyl but not for Charizard.
Hope I helped:)