Let's see some canon feats, shall we? I'll be using movie Thanos, Josh Brolin's acting is too good to pass up on an excuse to rewatch Infinity War clips.
Thanos:
Snapped away half of the universe
Kicked the Hulk's ass without using any Infinity Gems/Stones/whatever the fuck.
Kicked everyone's asses, pretty much.
For his best durability feat, I'll need to explain a little. Remember when Thor threw Stormbreaker into Thanos's chest, and he survived to kill half of everyone? Let's go ahead and calculate the sheer power of that attack using some good old-fashioned power-scaling using feats from Thor: Ragnorok.
We all know that Hela was weaker than Odin, and Surtur was stronger than Hela. So obviously, Surtur is about as strong as Odin, practically. Odin needed Thor to get rid of Mjolnir to get to his strongest level and surpass him, but already at that point Thor had absolutely wrecked Surtur. Therefore Thor needed to be stronger than Odin to be the king.
Thor got stronger when he got Stormbreaker, so it is safe to say that he was stronger than Odin, who was about as strong as Surtur, who was stronger than Hela. What is Surtur's best feat? Blowing up Asgard. The only question is, how big is Asgard?
I've come to the conclusion that it doesn't matter how big it is (it appears to be about the size of an island) but how durable it is. There's no way it wasn't enchanted with awesome Asgardian magic and shit, not to mention the special materials used to build. Also, it was free floating in space with what looks like from ground level multiple moons. This means its mass had to be insane, and its density even crazier seeing that it's pretty small.
All-in-all, with all things considered, I would say Surtur's feat of obliterating Asgard is planetary level, and that's a low ball estimate. And, as I've established, Thor>Surtur/Odin>Hela. That means that as the absolute lowest I can take this, Thor hit Thanos with enough force to destroy two planets, and Thanos took it and survived.
Thanos's Powers:
Capable of creating illusions
Shooting powerful energy blasts
Smart enough mind to manipulate the king of manipulation: Loki
Manipulating tangibility, even for other things
Creating portals
Turning people into spaghetti
Controlling minds
Control time
Cause anything to happen with a snap
Now for Shaggy, cue the maniacal laughter. His feats:
Easily defeating a room full of bikers
Eating entire buffets without a second thought
I'm only stating the blatant here, let's get into the meat and potatoes of Shaggy's common feats that I don't think people notice too much.
Ever notice how Shaggy not only keeps pace with a Great Dane, when he starts running he doesn't go at first?

Shaggy preparing to use his own patented fighting technique on Hitler to end WWII colorized
I have a theory for this. It's pretty simple. You know how if someone is sucked into the event horizon of a black hole, all the onlooker would be able to see would be their figure turning red and slowly disappearing? Well, Shaggy is recreating this effect with how fast he can run.
This means Shaggy is running close to the speed of light. This isn't counting the numerous quotes by the Mystery Gang of mortals being vaporized in Shaggy's presence. This means just by existing Shaggy constantly exerts a force of about 3 gigaojoules, which is enough to melt 5,000 pounds of steel or constantly emit the sheer force of a lightning bolt. This isn't even 1% of his power, he is actively containing it at this point. And we all know what lightning bolts did to Thanos.
Shaggy also knows how to use Ultra Instinct, and ability that grants him complete awareness of all that is around him and lets him react to anything without thinking. Now that I've gone over some of Shaggy's feats and powers, let us see how a fight would turn out.
FIGHT
Thanos set the final Infinity Stone into his gauntlet. It glowed brightly and he grinned. He was now a god. His plan would be enacted, half of all life would be eradicated. He prepared to snap when a voice sounded behind him.
"Hey, you don't wanna do that." said a green-shirted youth. He stepped forward. "Don't make me do it man." Thanos started to feel pain all over that grew slowly, engulfing his body. Grunting, he made his body intangible. Somehow he still felt it, but now it was no more than a little sting. He floated above the boy.
"You are a threat to my goal." Thanos said sadly. "Forgive me." He thrust his fist out and unleashed a purple beam from the Power Stone. Shaggy slowly disappeared as the blast ripped apart the entire forest they were in. His afterimage was still clear as he tapped on Thanos's shoulder and kicked him. Impossibly, the power of the attacks overwhelmed the intangibility and sent Thanos through a mountain.
"Well then." Thanos said as he got up. Shaggy was in front of him once more, but Thanos had slowed time to compensate for the speed difference. Their clash obliterated the mountain range. Thanos used the Time, Space, and Power Stones in tandem to stop Shaggy in space and time, then blast him into the moon. He brought the moon down to meet Shaggy, which slammed him back into the Earth.
The crust began to splinter, and the magma from the mantle seeped through. The planet was exploding. Shaggy smiled as he stood up in the center of the devastation. "This has gone on long enough." He shimmered a slight blue-grey color.
Thanos was raging now. Why wouldn't this child just die?! He snapped his fingers as Shaggy punched through his chest with a small statement that made Thanos die while frozen with unadulterated fear.
"This is only 1.000002% of my power."
The snap had been meant to erase Shaggy from existence, but he'd dodged the rip in space-time. Thanos's death would result in Shaggy being safe from the attack, as if there were any danger