If a single PC is scouting out ahead, they should be making a stealth check, not having an entire encounter all to themselves. Encounters can easily take 20+ minutes to resolve. That’s not to say that a PC who decides to sneak alone should magically be able to resolve entire encounters with one die roll. Rather, lone stealth PCs should be scouting only a little bit ahead of the rest of the party, while solving entire encounters by just stealthing past them should generally be discouraged unless the entire party can participate.
A stealth encounter revolves around two basic types of enemy: Guards and sentries. A guard is someone who can plausibly kill a stealthed party (and especially a lone sneaker), whether by themselves or as part of a group that can easily alert one another. For example, a single orc may not pose a challenge to a level 5 party, but if thirty of them are all patrolling in one giant pack, they’re dangerous, and they’re guards. Guards are usually near-blind, with no Perception training and unexceptional Wisdom. PCS with Stealth training and who are at least level 5 have good odds of sneaking past a guard.
A sentry is someone who can plausibly detect a stealthed party. This means they need to have enough Perception to keep up with the Stealth of at least a Ranger (trained in Stealth, high DEX build), and sometimes even a Rogue (expertise in Stealth, high DEX build). A sentry isn’t necessarily strong enough to pose any kind of serious threat to the party members they detect, and in fact a pure sentry should be very weak, weak enough that a party can assassinate them without having a long drawn out fight. This is the Rogue’s forte, between their sneak attack and their Stealth expertise, and especially if they’re the Assassin archetype and auto-crit in addition. The Rogue is the most able to sneak past a sentry long enough to assassinate them so that the rest of the party can pass safely.
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