Really depends on what you want the feel to be? I found boss solos to be disappointing because the tight level math. Saves for example, for a solo boss are gonna be so high you have a 10-20% chance of a spell landing. So, instead of going with really cool spells, you go with cantrips that might have a slightly inconvenient rider to slow the boss down. Maybe take out one of its three actions so it cant clobber your martial characters too badly. Speaking of which, since the crit system is <10>, it means that the boss is gonna hit on attacks, thats not even a question, but what is up for die roll is if its going to be a critical hit or not. Something, again, the PCs have a tiny chance of happening on their behalf.
What that does is cause a nerve wracking encounter where if the party doesnt work in mechanical unison like a volleyball team bumping, setting, spiking, then the party will be wiped. 5E, for example, works great under that paradigm, but isnt required to be successful. While I dont like the feel of it, it does make solos a capable combatant without any assistance like legendary actions or minions.