To give more details on Pachirisu and this Pachirisu in particular:
Pachirisu’s stats aren’t great. They aren’t terrible, but they aren’t great. Of the four Pokemon involved in this particular exchange, Salamence and Garchomp have a base stat total of 600, Mega Tyranitar has 700, and Pachirisu has 405. Yeah.
To make things worse, Pachirisu is a gimmick Pokemon of a class known as a “Pikachu clone”. Its entire purpose is to be a generic Electric-type with middling stats and chubby cheeks. It was never built for high-level play, and a lot of people will even say that it’s not worth it for casual because there are so many better Electric-types.
Pachirisu does have a couple of things going for it, though. Firstly, its Special Defence and Speed stats aren’t totally garbage, and its Defence isn’t bad either. This means that it can take a hit if you train it right (which Se Jun did).
Secondly, the moveset that this particular Pachirisu had was a brilliant one for what Se Jun wanted it to do. Nuzzle deals a tiny amount of damage but is guaranteed to paralyse anything that isn’t an Electric- or Ground-type. Paralysis halves speed and gives you a 50/50 chance of not being able to do anything that turn. That’s HUGE. This is a game in which most major attackers are fast as hell, and outspeeding is vital to pretty much all strategies.
Super Fang, as mentioned, is guaranteed to do 50% damage to anything that’s not a Ghost-type. Pachirisu cannot explicitly KO with this, but what it can do is break down its opponents’ defences. You can’t use inventory items in competitive play, so it’s unlikely that this damage will be healed, and 50% damage can make the difference between Pachirisu’s partner taking two moves to defeat its opponent vs taking one move.
Follow Me redirects opponents’ moves onto Pachirisu if they would have otherwise hit Pachirisu’s partner. This lead to the instance above, in which Pachirisu survived a hit from a Draco Meteor (a 130 power attack in a game where 90 power is considered excellent and 100 power or above generally requires major drawbacks). You might think “but if it couldn’t take out Pachirisu, then it couldn’t take out the Salamence it was meant for, right?” NOPE: Salamence is weak to Dragon-type attacks like Draco Meteor and so would have taken double damage. That’s probably a KO.
But the real purpose of Follow Me was for Pachirisu’s intended partner: Gyarados. Depending on the situation, Gyarados takes either double or quadruple damage from Electric-type attacks, which is a shame because Gyarados is otherwise terrifying. Pachirisu has the passive ability Volt Absorb, which means that it’s healed by Electric-type attacks rather than taking damage from them. So if you try to use an Electric-type attack on that Gyarados there’s a very good chance that you will end up healing Pachirisu instead. Not what you want, especially when this particular Pachirisu is so unbelievably bulky.
Oh, and its last move is Protect, which totally nullifies damage on Pachirisu for that turn. Useful if Pachirisu can’t do anything useful this turn but you don’t want to swap it out.
And that Sitrus Berry? Yeah, that thing recovers 25% of Pachirisu’s total HP the moment it goes below 30% total. So if you just barely fail to kill Pachirisu, it’s going to heal itself. Have fun.
The last thing Pachirisu has going for it, at least in this case, is intimidation. The universal response to this team was “Who the fuck brings a fucking Pachirisu to the World Championship?!” But then you realise that Se Jun is totally serious about this ridiculous plan. And then you realise that it’s working. And then you realise: oh shit, this guy is really good. He knows what he’s doing with this.
TL;DR: By all accounts Pachirisu should have been an utterly stupid thing to bring to this level of play, but in practise it was a very clever set-up that took advantage of its decent defensive stats and useful support/survival movepool.