I wasn't actually talking about USSJ as a feasible means of having a stand-up fight with an SSJ2 -- it isn't, not remotely in a single-fighter duel -- just clarifying how it works.
Bear in mind, I'm not talking about this being the only plan that works. This is one plan. It can work, because there's more than one Super Saiyan to chew through at any one time, everybody else knows to move immediately in this circumstance, and they all, base Saiyans included, are at, "traverse a planet in seconds," speeds. But it's not the only one that can work, it's not even the first one that would be tried, and it's not one that's terribly likely to work. I was using it as an example of something that can work, as a means of demonstrating that base Saiyans are not something that can be safely ignored in sufficient quantities.
Another one would be if a Super Saiyan 1 provoked the Super Saiyan 2 into a beam struggle, and then powered up somehow. Perhaps with USSJ, perhaps with the Kaio-Ken. Either way, at that point they'd be too strong for the enemy to safely turn aside -- even if just for a moment -- and they could hold out for long enough for the rest of the regular Super Saiyans to join in. And then a normal Saiyan could join in. And then another. And then maybe the ten thousand or so Oozaru who've trained to their absolute limits. And another base Saiyan, and another, and another, until all the attacks going into it were enough to take out the Super Saiyan 2.
Or a group of normal Saiyans go Oozaru and stall. They'd all die, but they would die slow enough to buy time, given that the form optimizes for durability and raw power. That would be enough time for at least two Super Saiyans to arrive, and they definitely could stall long enough to buy the other Super Saiyans and hordes of base Saiyans time to arrive.
Or the Super Saiyan 2 could get cocky, pick a fight with three thousand Saiyans as they held a party in the Training Hall, and miss just one single Kienzan in the middle of the storm of blasts being thrown at them.
Now, all of these have obvious failure points, but they all can succeed as well. They have the noted issue of going up against a monstrously strong opponent, but they're not totally infeasible. Most especially because -- remember -- we're not using the typically-accepted model of Super Saiyan forms being multiplicative. For my ease of use we're on BOD-standard, which holds them to be additive, and so SSJ2 doesn't multiply base strength by x100. It does double the Super Saiyan boost from FPSSJ by 2, but only the boost, not base power, so a Super Saiyan 2 who's just transformed for the first time isn't even twice as strong as a FPSSJ. They are decisively, even overwhelmingly stronger, and the transformation removes limits and allows even greater strength over the long term, but in the short term of the first transformation, which nobody on the planet could possibly miss, they're not to the point where they can take out one FPSSJ quickly enough to stop a second showing up picoseconds later, and then a third. Given that there are up to six Full-Powered Super Saiyans on Garenhuld at any given time, this adds up. And if they can stall for the heartbeat it would take to marshal the horde of base Saiyans -- which is likely -- the SSJ2 would have to contend with that. And by the math, you only need to muster a thousand Saiyans out of three hundred thousand to overcome a Super Saiyan 2.
Now, the SSJ2 could know Instant Transmission. They could also not. But for what that exact reason, there's never a time when there are no Scions, Lords, or ex-Lords capable of the same move, and it's impossible to hide that ki signature. At that point, it becomes a game of endurance. Who runs out of ki first: the pentet/sextet of people who can stay in their transformations all day because they've mastered them, or the one who just got their transformation and has had all of their progress on endurance training nullified because of it? They can't drop out of it -- six Super Saiyans pinging the planet with their full power will find nearly anything, and if the SSJ2 starts dropping their ki to hide, they abruptly become a squishy mortal for a window that is just big enough for the Super Saiyans to IT in and vaporize them in the moment of being strong enough to sense, but weak enough to be helpless.
Or, in the first three seconds of the engagement, the SSJ2 wipes out the Super Saiyans, and then vaporizes any base Saiyans that show up until the planet decides to collectively surrender.
My ultimate point is that a fight between an SSJ2 and a planet of million-unit-plus fighters is an uncertain prospect, enough so that the Lords are willing to throw their entire population into the meat grinder for the slightest hope of slowing the enemy down. They have many plans, all revolving around nullifying the speed advantage in order to bring the collective might of the Exiles to bear, which adds up swiftly. One of them always knows IT. One always knows the Kaio-Ken. All of them learn USSJ at some point if only so they can contest a beam struggle for long enough to get reinforcements. And yes, most of the plans bank on being able to provoke the enemy into unwise action, which is not as foolhardy as it sounds given that the transformation is -- quite literally and exactly -- legendarily hard to remain in control of. And all of this, these literal centuries of preparation and planning, have given them...maybe a forty percent chance of victory. Fifty, at a stretch. Given that they need to concentrate strength and quickly, they are at an inherent disadvantage given that you can't concentrate strength down any farther than a single person.
The reason I'm hitting the ways they could win so hard when they're technically in the minority is because I want to make it clear that we don't just say, "no, not possible," and go home. I want to privilege and reward clever thinking in this quest, and making clear that brute strength can be overcome is the biggest way to do that. Not that any means of doing so are without cost -- but they exist. Even if you take no clever plan and simply order your people to fill the skies with ki blasts so that there's nowhere to dodge, that will add up -- each one will only scratch the SSJ2, but there are over three hundred thousand scratches being fired at any given time, if you martial everybody, multiple times per second.
Not that that has a higher chance of success than perhaps five whole, shiny percentage points. But again, it's a way.
