FormerlyHemlock
Hero
In games where the pcs are too strong for deadly encounters per the guidelines, the best thing a dm can do to fix things is remove magic items, bring attribute scores back to reasonable levels and reduce the amount of advantage given. In other words, don't try to balance the game by unbalancing the monsters... do it by balancing the pcs.
Why not? 5E is a great edition for balancing things on the monster side. If your 8th level PCs are effectively 12th level due to magic items, feats, good tactics, or anything else (hirelings, poison, etc.), let them face 12th level threats. Right now I'm running 11th level PCs through a crashed beholder tyrant ship with 12 beholders, about 50 bugbears, and another 80-odd goblins. Not in one encounter of course, because the PCs are not idiots--but even in a best-case scenario they're probably looking at 28,000 XP worth of enemies (e.g. catch one beholder alone with 6 bugbear bodyguards), which is a 20th level Medium encounter. And it's working great! Tension is high and PCs are fully engaged with their A-game. They may die here and that is what makes the game fun. (BTW, the PCs aren't under a time constraint except for general "don't take longer than you need to or the enemy will entrench and start building alliances," which makes this scenario easier than it may appear to be. They are talking about bringing in siege weaponry for example. If so I am planning on having the beholders detect it and tunnel out there for a counter-ambush, using disintegration rays to dig and goblin sappers to shore up the tunnels.)
If the game is boring because PCs and challenges are mismatched, amp up the challenge, don't nerf the PCs! The DM is responsible for the world, not for the player characters. Players own PCs.