Caesar has Howitzers now. The fact that they're probably in the same ballpark of deadliness as a Saderan warrior in melee combat, with comparable equipment and armor as well (this is one of those cases where they're better equipped for melee then modern soldiers would be in a similar matchup) but also flush with guns thanks to their many conquests... and apparent trade agreements with the Gun Runners and Van Graffs, means they could run riot over the Saderans as long as the Gun Runners can keep them supplied with enough ammunition. And if the numbers aren't completely against their favor.
The Battles woudn't quite be as one sided though. The JSDF likely had way more artillery, and rocket launchers, and airpower, mines, barbed wire, trenches, and could throw as many bullets as they wanted downrange in their tens and thousands without needing to worry about running out. So the firepower advantage of the modern army versus Legion is still pretty vast.
The only problem is I doubt anyone in the Legion has the Pornomancer energy that Gate's main character has in creating a harem out of every fantasy character encountered that could be a threat to Japan's imperialist ambitions. Caesar is silver tongued, but he ain't some Cherchez le Femme or whatever. He's like your know it all Uncle. Gross.
Lanius... yeah whatever. He'd have a better chance trying to court Sergeant Kuribayashi.
One might assume Vulpes Inculta seems slender and effeminate enough to be the manboy warrior who can gather a harem but he's not nearly as unsure of himself to fulfill that role, the smug motherfucker.
Lucius is gay. Sorry, that's heh... head canon, but that's the vibe I'm getting.
Best chance is probably Aurelius of Phoenix. He's kinda boyish... but pretty masculine. But you know he has a softer side with all of those toys he collects and is probably like really melancholy and sad on the inside and just needs a harem of magic fantasy girls to open up to. He should lead the first advances through the Gate and establish the Beach Head... or else the Legion will be caught in the Long War which they may not have the resources to sustain, much less win. And even Aurelius isn't an "I Win" button due to his habit of crucifying profligates... of which there ARE MANY. Some of whom that will desire to mate with him!!!1
EDIT:
Actually Cursor Lucullus might be the best bet. He has that hidden main character energy (being a Courier) and is a cute boy yearning for some character growth. This Versus Debate could be his main character story arc.