Well, I hope you are not using the mercenary grenadiers from the diplomatic center, because those are useless against upgrade walls.
But, the regular grenadiers from the 18th century barracks, if fully upgraded on their attack ( especially the last one +1500 - 43000 gold) should make your grenadiers have the ability to brake down a wall in no more then 5- 6 grenades. It is especially this last upgrade that makes them efficient in breaking down walls. As long as that very last attack upgrade in the barracks has been researched, it can indeed take a looooooooooooooooooong time to brake down a wall.
In any case, archer mercenaries , as you suggest are not gonna do any better, mercenaries are useless against walls. They are just quick reserve troops, thats all.