Running a campaign right now where the planned BBEG is going to unleash a Tarrasque at some point. I think its a really cool monster, but balancing the encounter seems impossible. It's a CR 30 monster, even with a 4 man party at level 20 it seems impossible unless they come up with some kind of exploit.
Any ideas for fun scenarios that could help this be balanced? I'd rather avoid just throwing a ton of npcs into the mix but I suppose that could help.
As a high CR monster, the Tarrasque is not as much of a threat to high level characters as it might seem. Its main strengths are its high hit points, many immunities, and damage output. Other than this, it doesn't have much going for it. It has no spells, and its saving throws behind its legendary resistance are unimpressive. It lacks the ability to fly or deal with flying targets at all due to a lack of ranged attacks.
The 5th edition Tarrasque is often made fun of for how unimpressive it is considering the hype surrounding it. Many will point to a simple Clay Golem's complete immunity to the Tarrasque, or how a Black Dragon could defeat it single handedly. A party of prepared adventurers level 15 or above shouldn't have much problem dealing with the Tarrasque, though the battle will be long, slow, and many players will find themselves bored.
When you take into account the things a high level party is capable of, flying, dealing high sustained damage, high level magic, and the tools to solve a plethora of situations in incredible ways, the Tarrasque on its own is essentially an unintelligent dragon without wings or a breath weapon.
If you are still worried about your party's ability to defeat the Tarrasque, think of the combat as more of a chase scene. The Tarrasques' Siege Monster feature gets little attention, but it embodies the Tarrasque's role as a Kaiju style monster. The Tarrasque is an agent of destruction, and the combat could see the players struggling to stop it as it tries to destroy one or more cities, only taking swats at the players to knock them away and staying mobile to make their assault against it difficult.
The Tarrasque could grant the players opportunity attacks by moving through the streets of a city and more or less ignoring them until it's clear they pose a threat.
You could also introduce several different challenges along the Tarrasque's path of destruction, introduce innocents for the players to save from the rubble, doomsday cultists, or crumbling ruins to block their path.
The Tarrasque's attention need not be on the players 100% of the time, because it may not see them as a threat at first.
Parties full of characters with low constitution scores and a lack of Mass Heal or similar may find themselves intimidated by the Tarrasque's high to hit and seemingly high damage, and it might be overwhelming if focused. But as Jeremy Crawford has said in this video:
...we were looking at what are the most optimal, meaning, often the most dangerous things this monster could do round by round...that would justify that creature's CR.
A creature's challenge rating was calculated by playing the creature in an optimal way, so if you want a monster to be weaker, you can play it in a sub optimal way. Having the Tarrasque focused on causing structural damage is one way to play it in a sub optimal way, another is sub optimal targeting. Instead of focusing on a character with low hit points until they die, a Tarrasque could focus on spreading damage out to tougher combatants. In my experience, spreading out damage with a strong monster gives players more time to react to the damage they are taking, while increasing the tension as their hit points slowly dwindle.
