How do I telegraph that an encounter that was too difficult before is now achievable?

Question

Assuming you used the techniques from How can DMs effectively telegraph specific dangers in D&D? and How can I make my PCs flee? to telegraph danger and the need to improve the characters skills/level before they can face the encounter.

How do you show players, without meta-gaming, that they are now ready to face said danger?

There are some examples where the players themselves recognize that they can now circumvent the difficulty of the encounter. For example (in D&D 5e), a Cleric that just learned Greater Restoration, which can cure petrification, will probably get the idea that the Medusa fight is not that dangerous anymore.

Other examples like an Adult Dragon that earlier munched on an adventuring party with magic gear do not have such "triggers". The Dragon is still way bigger than them and the players did not know the exact level of the adventuring party.

Answer 1

Give them a better idea of the threat level

If you are playing a sandbox style game, it is up to the players to determine what level of known risks they wish to face. Thus, you probably do not want to telegraph too strongly that you think they are ready; that decision is up to them. Dice have vagaries and players play with differing levels of tactical skill and system mastery and character optimization. Unless you plan on using a deus ex machina to put your thumb on the scale for one side or the other, you only know how deadly an encounter is meant to be, not how deadly it will actually be. So, in a sandbox style game, let them figure out how much risk to take. Just give them a fair idea of what those risks (and possible rewards) are.

In your dragon example, they can meet someone that can tell them more about that specific dragon's capabilities and thus judge for themselves if they wish to face it. One indirect way to do that is to tell them more about the last adventuring party so they can say "Yes, we are more powerful than that now".

Put the encounter on a timer

One way to encourage the encounter is to do something to make it urgent. If something bad happens if the issue is not resolved within a time limit, they will be much more motivated to find a way to resolve it now. From any but the most ruthless or most dedicated to a "breathing world" GM, this will also be a not-so-subtle but also not meta-gaming signal that the GM thinks the encounter can be resolved within that time period.

Perhaps the dragon is now threatening a town by demanding a major payment of tribute within a week or it will attack. The adventurers now have a deadline to resolve it or the town will be directly harmed. They won't even be able to say that they can get the tribute back from the dragon later when they are yet more powerful, if much of the tribute is in the form of consumables like cattle that will be eaten. Or, if you want to be more cliché, perhaps the demanded tribute includes a hostage.

Answer 2

Let them demonstrate their new skills

This is a really fun way to show, not tell their development growth. Sure that +3 in strength sounds nice, but one of those bad boys crawled too far our from the camp, time to see how strong this axe really is. Let the crimson rain be the judge! This also has the added benefit of scaling the 'boss' as well. If your players feel strong now, you can always smack them down with a single stronger enemy, play with their confidence a bit, just be careful to give them a chance to escape if you think a party kill has a high chance to occur.

Make it personal

This is more niche, and a motivator / MacGuffin, but after the team has been training or acquiring a new skill or what-have-you, and you believe they are ready for the big bad, you can always have the barkeep get kidnapped! Now the team is faced with a choice: revel in their new power to take down the big bad, or lose their dearest friend and quest giver forever. This also leads to great double crosses if your story allows it (e.g., the barkeep gets kidnapped by an evil grand wizard, but turns out he was the wizard causing the trouble all along, oh no!)

Answer 3

Give Them No Choice

Corner the party with the enemy in question. Most groups I've been with, especially in D&D, would rather try to take it on than just sit there and get eaten. Other examples include needing a specific MacGuffin and the Big Bad won't give it up, and the Big Bad is already attacking.

The Power of Meta Through Narrative

Narrative is a crucial tool. Drop hints about how certain features of the Big Bad don't scare them anymore. You said "without meta-gaming", which would negate telling them to just go do it already. However, there are ways that are equally effective and not nearly as direct. My players tend to doubt whatever I put in front of them which is how I avoid the 'Murder Hobo' issue. When I am pushing for a fight, I try to prime them.

"The last time you stood frozen when the dragon spewed its fire. This time you are poised."

"When you met before, striking its scaly hide sent violent shudders through your arm. During the last battle you fought, your blade could glance from armor and sunder shield"

"A fireball that barely scorched the side of a barn is now reducing whole wagons to ashes"

Direct influence from NPCs "I bet you are strong enough to take on the Big Bad now!"

Or even a simple "wow, I remember when that was only a +2"

Propping their confidence is usually instrumental with encouraging the task. So especially if you know a big challenge is coming, start planting seeds.

Answer 4

Make it both very obvious and very specialized.

If party was almost destroyed by a monster turtle before, let them find The Cleaver of Tortoise Demise, +10 against carapaces. You may make it break after they kill the turtle. If they find a key to the door that was locked, it should almost have a label "From that door, you know, there".

If you want a monster that the party would overcome in time, but not now, make the monster exploit the lack of some specific move that the heroes will have, but not now. The most obvious example is: make monsters have a really huge dodge bonus to AC against the first attack in turn, low health, and play them cleverly enough so that the party can not pile on them one by one. Such monsters will stop low-level party, but as soon as heroes learn double attacks, they will mow down those monsters no problem.

The concept of "beef gate" is hard to implement because some players, especially with most background in video games, think that if you meet an enemy, you must defeat it. Retreat is not their option because they are accustomed to every threat being defeatable. So be ready that some players will fight to death and then be angry that their hero actually died.