I define "not a problem" or "without difficulty" in an encounter based on how much of my party resources I use; hit points, potions, magic items, spells memorized, and so forth and so on. If I didn't use all that much resources its an easy encounter. If it took everything I have then it was a difficult encounter.
It doesn't matter if you are 1st level of 20th level. In my last play I was doing the whole Paladin/Cleric/Champion of Torm thing and well, the Dracolich wasn't a problem. The demilich and its defenders were.