Ice and other dragon types. But also here's a good tip! When in doubt try ghost. Pretty awesome.
It is ice Pokemon
Unfortunately, fighting Pokemon are not super-effective against dragon-type Pokemon. However, if the fighting-type (move or Pokemon) is strong enough, it can still do substantial damage against a dragon-type Pokemon.
Dragon type moves are only super-effective against other dragon Pokemon. However, if the dragon type move is on a dragon-type Pokemon, the move will do substantial damage to many different Pokemon. This is because, dragon-type Pokemon usually have outstanding stats which make them stronger than other Pokemon.
Any Ice type Pokemon with a Ice type move because Ice moves are super effective against Dragon and Flying types and Altaria is a Dragon Flying so that Ice move will multiply its power by 4! Here are some ice type Pokemon: Glalie and Walrien.
Draco meteor
Any Pokemon that knows a rock, dragon, or ice move. Those are the things Salamence is super effective against.
Dragon Pokemon are weak to ice and dragon type moves, and resistant to electric, fire, grass, and water type moves (By resistant I mean that when those type moves hit a dragon Pokemon, they will only do half the damage that they normally would). (This answer only applies to pure dragon Pokemon. Extra types such as flying/dragon, will have more weaknesses and strengths.)
When fighting pokemon with low HP Dragon Rage is a great move as it is a Set-Damage Move meaning it will always inflict the same amount of damage no matter what. Dragon Rage will always inflict 40 HP Damage regardless of the pokemon's stats.
any dragon Pokemon
If I were battling Lance, I would use Electric and Dragon Pokemon but I would use thunder because although dragon is useful against dragon types it will backfire if Lance uses a powerful dragon type move. If you are re-battling Lance use Electric type moves against all of Lance Pokemon except Garchomp because Electric type moves don't have any effect against Ground Type moves, but Dragon type moves would be the most effective type of move to use.
Any Pokemon that has a dragon move and is not a ground or rock or dragon or fire type.
Steel and Ice. A: Or use a Pokemon with a dragon type move
I think he should lean dragon pulse, earthquake, flamethrower, and fly. these all do more then 90 damage so that's good. Plus earthquake is good to use against rock and electric types, flamethrower is good on ice Pokemon. and dragon pulse is good against dragon. These types are all super effective to you so its good to teach then attacks to use on them. Or you could teach him dragon claw/outrage fire-blast/blizzard surf/thunder extremespeed