As Aerodactyl is the only Pokémon with Ancient Power on Lance's team, I'll assume he's the sole Pokémon you have trouble with. Though, my suggestions should still be able to help against the rest of his team.
Aerodactyl is one of the fastest Pokémon available in Kanto, though with its typing, it comes with multiple middling weaknesses, those being Water, Rock, Electric, and Ice. At this point of the game, you'll have access to Surf through HM, Rock Slide through Move Tutor (if you've yet to teach a Pokémon that move), and likely enough money to afford a few Thunderbolt and Ice Beam TMs from the Game Corner.
Generally speaking, a good Pokémon to take on Aerodactyl is one with enough bulk to tank a hit from it, as well as being strong enough to deal significant damage to it with one of these moves. Some Pokémon that fit this criteria include Zapdos, Vaporeon, Nidoking, Clefable, and Snorlax, which are all Pokémon common in a Fr/Lg playthrough that you may be using already. I wouldn't recommend swapping out members of your team for these Pokémon (besides Zapdos, which is obtained at level 50), as they are likely the highest level you have, and would require more training to defeat Lance than your current team.
Assuming you don't have any Pokémon that can learn any of the moves mentioned, I'd recommend resorting to training up your Pokémon by rematching the Elite Four until they eventually wind up beating Lance.
