Numbers are a key component in 5E. During the early release, there were long discussions about a village of commoners able to kill an Ancient Red Dragon just due to BA. They will die by the truckloads, but ignoring any sense of morale they will eventually overwhelm and kill the dragon.
The CR system is fine to use as a measure of how powerful each creature is relative to each other, but the encounter guidelines are not very useful, except to new DMs. Experience will give you a far better clue as to encounter design. From personal experience, I've thrown Deadly x3 encounters at my party they shrugged off with ease. The same group faced a Hard encounter of two adult dragons (in lair with legendary actions) and it was close to a TPK.
She would fire from cover (at the intersection), then move back to the next intersection, jumping over the pits. If the party tried to chase after her, they couldn't avoid her gaze (otherwise they couldn't know where she went), and the usually fell into the pit (Dex save). The medusa had something on her person they needed, so they couldn't even just avoid her; they needed to kill her (or at least get her to surrender). Due to the level there was little danger of anyone being turned to stone, but several times it was close, and eventually it caught up to them and the barbarian did become petrified before the end.
tl;dr: smart design can make single monsters far more dangerous than they should be, but that is the exception, rather than the rule.
The CR system is fine to use as a measure of how powerful each creature is relative to each other, but the encounter guidelines are not very useful, except to new DMs. Experience will give you a far better clue as to encounter design. From personal experience, I've thrown Deadly x3 encounters at my party they shrugged off with ease. The same group faced a Hard encounter of two adult dragons (in lair with legendary actions) and it was close to a TPK.
Very true. I ran a single medusa against my party, which should have been an Easy encounter. However, I designed her lair to take full advantage of her abilities. It was a maze of irregular passageways, with T intersections about every 30'. Between each intersection (except a single path through the maze) had a hidden pit trap, and I'd given the Medusa Boots of Striding and Springing.Solo encounters need to play smarter, not just bigger stats.
She would fire from cover (at the intersection), then move back to the next intersection, jumping over the pits. If the party tried to chase after her, they couldn't avoid her gaze (otherwise they couldn't know where she went), and the usually fell into the pit (Dex save). The medusa had something on her person they needed, so they couldn't even just avoid her; they needed to kill her (or at least get her to surrender). Due to the level there was little danger of anyone being turned to stone, but several times it was close, and eventually it caught up to them and the barbarian did become petrified before the end.
tl;dr: smart design can make single monsters far more dangerous than they should be, but that is the exception, rather than the rule.