Marill evolves into Azumarill at level 18.
Marill will evolve to Azumarill at level 18
Catch it
Marill does not need a evolutionary stone to evolve into Azumarill. Marill will naturally evolve into Azumarill at level 18.
it doesn't the stages are azurill marill azumarill
Azumarill doesn't evolve according to level in Pokémon HeartGold because it's not capable of evolving at all.
A Level 25 Marill can still evolve into Azumarill even despite the fact that Marill evolves into Azumarill at Level 18, all you have to do is level it up and it'll evolve into Azumarill as long as it isn't holding an Everstone.
Evolve Marill @ level 18.
Marill evolves into Azumarill at level 18.
Marill evolves into Azumarill at level 18.
Marill evolves into azumarill at level 18
Find a Marill in the wild and evolve it into an Azumarill. I think Marill evolves around level 25. marill evolves at level 18 in sapphire