How to Challenge my Monk?

So I'm DMing a party of fairly fighty characters. The standard part is:

Halfling Way of Shadow Monk

Wood-Elf Totem Warrior Barbarian

Human Oathbreaker Paladin/Dark One Warlock

Half-Elf Battle Master Fighter

(Coming Soon): Goliath Swashbuckler Rogue

Meanwhile, temporary characters include:

Tiefling Eldritch Knight Fighter

Human Knowledge Cleric

So there's the average party group for you. As you can see, almost all fighty characters (even the Goliath), but in most cases, I can give reliable challenges. Now, we rolled for stats, and what happened is the monk turned out just a little bit too powerful. Now, after experiencing this, a few levels on, there's next to no way to kill her, or really challenge her in any way.

The party is level 6. She has AC 19, +8 to hit, and some positive number in every stat barring Charisma. At this juncture, smart enemies will either mob her or mob her friends until she's the only one left, and this feels like a horribly cheap fight. How do I challenge this monk in combat (or failing that) out of combat?