By 7th through 9th level, a single monster falls fast to focused fire. That's expected and fine. Real challenge emerges from layering threats together.
Multiple foes change the math entirely. A frost giant commanding two lieutenants and a wolf pack forces players to manage targets, not obliterate one thing. Layered threats hit differently too - giants hurling boulders while shamans cast hold person means players choose which threat to address first, and something always gets through.
Environmental strain adds pressure that numbers alone can't. Icy cliffs create positioning hazards. Avalanche risk forces saves. Exhaustion from harsh conditions compounds damage. Time pressure sharpens everything - when that ritual completes at dawn, there's no long rest to reset, so every resource spent now matters.
Your job at high levels isn't designing one unbeatable monster. It's combining threats so players feel genuinely stretched across multiple fronts.