How can I make recurring villains/bosses actually threatening?

What the title says, I want players to face off against recurring villains and foes, but I usually have the same problem. If I pit them off against the entire crew solo, they just get demolished, and if I add a few mooks, the mooks get demolished first, then the boss gets annihilated.

When it comes to recurring villains, they usually die before they can escape, and rarely feel like a threat. What can I do to make enemies feel like more of a threat, without necessarily increasing their quantity in a fight, or making them able to one-shot party members?