Yep, I'm gonna throw one of these at my when they're still level 4, fools gold style

TOTALLY INSANE AND UNKILLABLE! LIKE WHY WOULD THEY EVEN INVENT THIS?!

The target of Contagion becomes Poisoned, and while they're under that condition a slew of other potential effects hit them, assuming certain conditions are met. The Tarrasque is immune to the Poisoned condition, ergo it cannot be affected by it, ergo the rest of the spell does not affect it, because those conditions won't proc. I suggest casting Grease instead, or getting a Rune Knight on Enlarge to grapple it, since the Tarrasque will have less in Athletics than an optimally built Fighter (10 versus 17, plus advantage) and legendary resistances cannot be used to bypass skill checks. You'll still have an angry Tarrasque in a headlock, but it was worth living for.

Technically if the level one character has a flying speed greater than the tarrasque's highest jump they can hover 15ft above him and use ranged attacks

This thing is so powerful... It's like a titan but x999,999,999,999% more powerful.

If i could describe this freak of nature... My 2 words would be this:               WORLD DESTROYER!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

TPK

Quote from BartFitzgerald >>

The target of Contagion becomes Poisoned, and while they're under that condition a slew of other potential effects hit them, assuming certain conditions are met. The Tarrasque is immune to the Poisoned condition, ergo it cannot be affected by it, ergo the rest of the spell does not affect it, because those conditions won't proc. I suggest casting Grease instead, or getting a Rune Knight on Enlarge to grapple it, since the Tarrasque will have less in Athletics than an optimally built Fighter (10 versus 17, plus advantage) and legendary resistances cannot be used to bypass skill checks. You'll still have an angry Tarrasque in a headlock, but it was worth living for.

 We do not need to "assume certain conditions were met" 

The other effects dont need to happen only "while they're under that condition"

The spell does not depend on it being poisoned

If someone hits it with a flame tongue, do we assume that even the slashing damage fails because it is immune to the fire damage

No, we don't

so too the spell will work even if it's immune to the poisoned condition

the ring of animal influence can be used to make this scared of you lol

4

my players thought my campaign was too easy... let's see how they like it now!

It's ME! - Tarasuqe2897

Unbeatable monster? Pfff.  Many wrote before me that flying and ranged/spell attacks would make a quick work on it as it is dangerous only in melee. But I would say that even a lone monk with the mobile feat and  haste could slowly beat it to death. Okay maybe 4-5 haste is needed or a Boots of Speed.

This thing needs a massive rework to be worthy of being called a CR 30 Legendary.  For campaigns that end at level 10 it’s scary.  For games that actually get to level 17+ it’s a joke.

Admittedly you need to have both a degree of reading comprehension and understanding of how wording works in 5e to get this, but I'll explain it again, just this last time and in the plainest of terms, step by step.
1.) "Your touch inflicts disease. Make a melee spell attack against a creature within your reach. On a hit, the target is poisoned." The Tarrasque is immune to the Poisoned condition, so it is not Poisoned, even if you hit it.
2.) "At the end of each of the poisoned target’s turns..." The Tarrasque is not Poisoned, because it is immune to the Poisoned condition.
That's it.

Even if the Tarrasque was not immune to the effects of this spell - which, it is - you wouldn't really achieve much, since it has a +10 to Constitution saves and advantage on the saving throws, since it is a spell. But that's neither here nor there, since the Tarrasque is unaffected by the spell. In case you're still unsure I encourage you to look online.
In short, we need to assume the Poisoned condition is met because the spell specifically states so, the other effects only happen while they are Poisoned because the spell specifically states so, the spell indeed depends on the target being poisoned (virtually the first the first ten words of the second line of the description).

Your Flame Tongue analogy is both inapplicable and used wrong, but let me spin it for you. If you fail to hit with the attack (counterpart of the Poisoned condition) you won't deal the extra fire damage (the diseases). In case you're still unsure about it for whatever reason I welcome you to check this online. You will find - over and over again - that you are regrettably wrong. 5e uses specific wording for a reason, learn to read it.

a clay golem can easily defeat this dude because a clay golem is immune to all the types of damage the tarrasque gives

Quote from LuxeryDND >>

If I were to describe a Tarrasque, I would be calling it away more overpowered version of Godzilla.

 To my players. "Imagine Godzilla. He was out on a pub crawl the night before and he's woken up this morning with a SCREAMING hangover. He reaches for the Advil but instead accidentally grabs the PCP and Meth and takes them instead. Just as the drugs are kicking in and the headache has hit Brobdingnagian proportions, the neighbor kids decided to play some Death Metal on their dad's stereo and they just cranked it to max volume just as he steps on a Lego factory."

For some reason you can have it as a pet!

Tarrasque: well, see you later tiamat. I've got people to meet and places to eat

As i am writing this, our DM is making us (4 lv10) fight a tarrasque with one magic weapon each (Melee), we are all screwed.

Technically speaking. A level 5 sorcerer with 2 level 9 spell scrolls can get extremely lucky and roll 20s on all beams of scorching ray. Because of the low dexterity the sorcerer has a chance of going first. The character can also use 5 sorcery points (Which they have exactly) and use quickened and Twinned sorcery points.

What's more realistic though is just a lvl 17 sorcerer using sorcerer points to get back a 9th level spell. Especially if the character has gotten above 20 charisma so that many of the beams hit. If not a one shot, most definitely a second shot if you were to get rid of some higher level spell slots. 

I’ve kill so many horny bards with the tarrasque