@mcv

1) single enemies work badly in D&D and often lead to anticlimaxes
2) use your legendary and lair actions to spice up the fight more than hurt
3) let them be 14 and do some Hit Thing With Axe, it's fun!
4) Listen to them make characters and see which subsystems they like, be familiar and make them have cool things hidden inside (someone likes hiding and sneakattacking, clump of stalagmites that they can hide in, someone likes fireball, group of like 10 kobolds doing a ritual that will become a big problem if not for a fireball)

And ya know, D&D's best adjective is *rollicking* not challenge.