Solo opponents can be solo and not solo at the same time. Consider these minions that are built into the solo humanoid's CR:
- Animal Companions
- Special Mounts
Your dragon knight for example may have a giant lizard they're riding around on. Now that "solo" NPC can take certain Move actions using the mount's Move in place of their own; this means they can be firing arrows from afar, then charge and still have actions, and even make a Ride check to bolster their own defense.
Another idea is to incorporate extra monsters but in a sneakier way. Consider the lowly zombie:
You spy a shambling corpse of the restless dead staggering toward you preceded by the stench of the grave. As it draws closer you notice a strange slick of ooze drooling from the sagging rot where once it's mouth had been.
The zombie has an alchemical ooze swarm sloshing around inside it, hitching a ride until prey can be found. This ooze swarm can be whichever variety you want and it only reveals itself when the creature makes a slam attack or is hit in melee. You don't have to describe it as a second creature if you want the fight to appear as an epic solo battle:
As you surround the rotten undead the fighter's axe bites deep into it's putrid flesh. A terrible reek issues from the gelatinized innards of the creature which issue forth in a nauseating spray as if water from a ruined dam. Fighter, give me a Fort save...
Point being now you have 2 attacks and 2 separate threats for the price of describing ONE monster.
Finally there are traps and hazards. The APL you're trying to hit is 5 right? Imagine a winter witch for the water element, and she's only level 6, however she's surrounded by a sheet of ice that is dangerously fragile in places. Beneath the ice are deep crevasses (CR 1 20' pit traps) which she glides across easily using some hex or power she has. The PCs in the meantime have to navigate an ice field making balance checks or waste resources flying and such, while also avoiding the pits that confound their attempts to gang up and melee/flank with the witch.
There are walls, fogs, trees, fire gouts, frozen terrain and still more that can rob PCs of options or help defend your solo. There are also traps, clouds of poison gas, pools of acid, and other things to threaten your PCs while your solo is fine. Finally consider things like fast-flowing streams, howling wind, driving rain, or intense heat to cause fatigue, minor ongoing damage, or debuff the PCs.