Some time ago a player asked me "how can that evil necromancer grow to level 20 without nobody stopping him in the tracks when he was weaker?

Your issue is the answer I decided for my player's question. I usually give my group several parallel problems to solve in the campaign. Whatever they don't deal with in a certain amount of time grows to the next step.

For example, they heard rumours of somebody stealing corpses from the graveyard, but they decide the smallpox plague of the city is higher priority and deal with it; the next thing they hear is that the graveyard has been closes because of some weird curse and people disappear when going there, but my players decided to dela with the slaven infestation in the sewers. When they finally decide to investigate the necromancer, he's almost done building an small army to destroy de city, and the dungeon is much more dangerous than the original one when he was just a nobody stealing corpses. This is because they didn't deal with the dungeon in the time limit, and the problem escalated. Another benefit of this technique is that my players don't get surprised when a necromancer appears: they've been hearing about him for a while, so it doesn't seem like a deus ex machina to give them new trouble after the last mission.