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First Post
Sure. The 5e solution for "not so good at that task" is disadvantage. So give the tarrasque disadvantage to hit with an improvised boulder it's tossing at a flying target. It still has a +16 to +19 to attack with a thrown improvised weapon (rock), disadvantage makes that roughly +11 to +14 to attack with the boulder.
It will still kill that lower-level target before it can kill the tarrasque with something minor like arrows or acid or alchemist fire or a cantrip, which is really the only situation we're trying to address.
If instead we're talking about a high level flying wizard shooting fireballs and bolts of real power, then OK that wizard should have a shot at beating the thing. But the low level archer or shlub with buckets of acid? No.
First off, saying "well the tarrasque is bad at throwing so he has disadvantage" doesn't work very well. As you point out, even with disadvantage the tarrasque will still be pretty accurate when throwing stuff. I'm saying that in my vision of the tarrasque, the tarrasque is NOT good at throwing stuff. That's the whole point. I would say the tarrasque throwing something is more like I would roll a die to figure out where it lands, and you need to save to avoid damage. It certainly wouldn't be an attack roll made at +16, with or without disadvantage.
Now, you don't have to interpret it my way. Your tarrasque might be a skilled thrower. That just doesn't fit my idea of how the tarrasque works though, and I don't think that my interpretation is deeply unreasonable.
Also, I would argue that a high-level wizard being able to solo a tarrasque is almost as big a problem as a low-level wizard soloing a tarrasque. In actual play, few low-level wizards are going to be fighting the tarrasque, so whether or not they can do it isn't that big a deal. The bigger problem is that what is supposed to be a major campaign-ending behemoth can potentially be trivialized by a single level 20 character. No single level 20 character, all on their own, should be able to solo the tarrasque with little risk to themselves. If they can, then the tarrasque doesn't really work as it seems like it should.
Here are the ways I would deal with the tarrasque:
1) Not use the tarrasque. Figure that a non-ranged brute just isn't an appropriate challenge for high level characters.
2) Give the tarrasque a ranged attack, possibly a breath attack sort of ability. That would knock pesky flyers out of the air.
3) Give the tarrasque regeneration and/or some sort of damage reduction. Or more abilities to punish ranged attacks. New defensive capabilities to weaken gnat tactics to the point of uselessness.