Originally posted by deathslash
Viltrumites have what are known as "smart atoms". It makes them more durable and is largely what enables them to survive well, pretty much anything. It's only when they take enough damage that their atoms begin to weaken and become more destabized.
https://static.comicvine.com/uploads/original/14/147508/4448544-capture1.png
https://static.comicvine.com/uploads/original/14/147508/4448545-capture2.png
https://static.comicvine.com/uploads/original/14/147508/4448546-capture3.png
Their smart atoms also allow them to become as strong as they need to be and their strength often times is several levels above their durability. This is why Viltrumites seem to be so "squishy" by the standards of others.
That's not to say that they're not durable though.
https://static.comicvine.com/uploads/original/14/147508/4531035-invincible+feat+durability+%2811%29.png
It takes quite a bit of durability to not only survive destruction of a planet (which was which was 1.25 the density of Earth) but to also shrug of the shockwave of its destruction.
And as far as heat goes, Mark (who was significantly beaneath Thragg at the time of these feats) tanked an explosion that turned Las Vegas into glass.
https://static.comicvine.com/uploads/original/14/147508/4414304-invincible+feat+durability+%282%29.png
https://static.comicvine.com/uploads/original/14/147508/4414305-invincible+feat+durability+%283%29.png
https://static.comicvine.com/uploads/original/14/147508/4414306-invincible+feat+durability+%284%29.png
And he also tanked a nuke that was designed for blowing up the sun.
https://static.comicvine.com/uploads/original/14/147508/4531055-invincible+feat+durability+%286%29.png
https://static.comicvine.com/uploads/original/14/147508/4531058-invincible+feat+durability+%287%29.png
These last two scans are by far the most informative.
https://static.comicvine.com/uploads/original/14/147508/4390479-capturee.png
https://static.comicvine.com/uploads/original/14/147508/4390474-capture.png
TL;DR: There's an upper limit to the amount of damage that smart atoms can receive before the begin to weaken and there's also an upper limit to the amount of heat that a Viltrumite can take before their smart atoms get destabized. It's a written in weakness the same way that Wonder woman is on par with Superman but can be hurt by bullets. The same way that Superman can be killed by Kryptonite. The same way that wolverine can die if he suffocates.
I actually did read all the text, but not the scans.
Other than the bio. Thanks though, I plan on checking through the scans later, as I don't know a lot about the character. It is sounding like it wasn't a bad showing given how their powers work.
Originally posted by Stoic
Hawkeye's arrows were the downfall of Krona as well. Let's all get out our low balls.
Didn't he just take out a machine with a dynamite arrow to foil his plans, making this irrelevant to piercing ability? IIRC as an homage to when Hawkeye did the same to Collector.
Also I'm noting, at least at the time DC VS MARVEL was canon, but even if forum rules didn't disallow it, I don't see poor piercing durability being a factor unless Thragg uses blades. Which, given I hardly know the character it could be.
cry more 
that was only after fighting Nolan, Robot, and a shit ton of other Viltrumites. I like how you also ignore that he was hit by several solar flares during that fight and kept going. Thor's hammer has also melted in the sun and Superman has even been affected by the sun before. But sure, let's pretend that the sun has any bearing on this fight.
