considering all soldiers of the EDF come with infinite ammo as standard (with minimal exemptions) i think any of them could deal it with enough time and inclination. BUT: you wouldn't come to me with this question if you didn't want to really get into the weeds with my Special Interest

Rangers could suffer with a lot of their weapons being single target so that more or less negates their small arms, considering ants are traditionally not very big. their wide access to explosives however, either via rocket launchers, multi-target guided missiles, or even the notoriously friendly firing 20 barrelled bouncy grenade launchers would clean up. as of EDF6 they can also field sentry guns which i imagine would eliminate any ant stupid enough to try sneaking in from the back. you could probably also run over a few with a tank

Wing Divers might have a harder time with their limited energy pool, but they also have access to the big explosions and weapons that can churn through entire crowds - either mirage class homing weapons or lightning guns. if the invasion happens to take place in a hallway you could bounce a thunder crossbow down there and take out the entire thing singlehandedly. not to mention their access to grenades which will float in place and spew out a thousand homing shots. you could probably use a laser rapier as a sweeping brush

Air Raiders would have it easy if you don't care about collateral damage since you can just carpet bomb or cruise missile the target area until glassed, or you could use your doomba if you want to get silly with it. Their catch is that with their reliance on kills generating charges for their more powerful strikes, would ants provide enough credit? even if there is a lot of them, an ant isn't going to fuel your orbital laser cannon very much, and then you're stuck with trying to use a limpet gun to blow them up very slowly until you can napalm the place again. could be a slog depending on how efficient you can be. calling in an E651 Titan tank for it would be very funny though. now that's pure decadence

Fencers (naturally) also have access to huge weaponry but some of it relies on other classes providing targeting beacons, which is inconvenient, and generally not worth it. their dispersion mortars, flame revolvers, and in a pinch melee weapons would do the job though. i would risk saying they could have the most difficult time of it, but there are some demented fencer players out there who can play that class like a wasp in a phonebox. insane mobility and potential for carnage, however i am not one of those players, tragically

hope this helps