It is not so hard for a group of mooks to roll a 15-16 which is the highest they would need to hit my player with the highest AC. Couple with the fact I often pre-roll large groups and it even easier when it is easy for a large group of mooks to get advantage on attacks, we haven't had any issue with running the large groups of monsters needed to challenge a high level group.
Can I ask about your pre-rolling method? What do you do, exactly?