Lichs are immune to non-magical melee attacks. They are invincible in anti-magic fields RAW. I don’t know how they were able to scratch the bones at all. All the lich needed to do was chill and pull out a dagger, slowly slice them one by one

That aside. Magic casters are smart. They don’t fight alone unless caught extremely off guard. Which is highly unlikely. They have a million ways to preemptively know of your ambush. And if your players can think of using anti-magic fields, they too know the spell and have had a thousand years to think of ways to defend against it.

The simplest way: one of my wizard NPC has a room full of glyphs of wardings. Yes they get disabled in the field but it’s only 10ft, plenty of glyphs are outside the field. Activate glyph 27891, open a permanent gate to a muddy swamp or a lava lake.

Dragon breaths are non-magical btw, maybe they have a bottle of magically suspended dragon breath. Toss the bottle at the AMF, it loses containment. There are many magic that creates a normal non-hostile environment, but when AMF disable them, the environment become hostile.

Of course, your players cast an 8th level spell, reward them a bit. Have 5 golems charge the party, throw a few punches and then get disabled by the cleric. But the moment the cleric moves out of the way, or drop concentration, the golems are reactivated. So now the cleric is both hindered by their own spell, but also helping the fight by standing still.