Nahiri is up there, but I did forget about Jaya, who was also an oldwalker and makes Chandra look like a kid with a box of matches.
Karn's pretty beefy, and even post-mending he still has his op-as-shit time reset power which is basically a middle finger to any conflict he's involved in. He's also functionally invincible, as iirc his silver body just kind of ignores physical damage.
And while you're correct, enough ants CAN take down a single planeswalker, they're not just dealing with 30-odd walkers, they're also fighting every citizen and guild-mate on Ravnica in addition to the broken-as-hell guild leaders, some wildlife, a native GOD, and at least one tree.
This is literally the plot of the first Avengers movie, with Bolas/Loki and his army of redshirt eternals/aliens invading Ravnica/New York. The only difference here is that instead of fighting like, 6 superheroes, the scrubs are fighting something like 30 superheroes AND like a million cops with guns, tanks, and rocket launchers. This isn't a fight, this is a one-sided slaughter. The only way this could be slightly nudged in Bolas' favor is if he manages to pull some actual Phyrexians or Slivers through the planar gate, because his little zombie army isn't going to cut it on Ravnica of all fucking places. If he invaded some backwater world like Tarkir or Innistrad he might have had a chance, but the sheer density of powerful characters on Ravnica even without Planeswalkers basically guarantees defeat for him, which is silly because this is the kind of thing he should have known.
Really, the only plane he could have invaded where he would have had an even WORSE time is New Phyrexia or maybe Dominaria. Invading Ravnica, which is full to the brim with powerful people, has got to be the Magic equivalent of invading Russia mid-winter. And the Russians have access to a superdemon.