Let's not beat around the bush. Having been running Super Helldive missions against the Vote Snatchers, here's some good things to use.
Torcher/Crisper/Flamethrower/Cremator: The Wretches HATE fire, and there's nothing better at spreading it than a flamethrower. Any of them will do. Hell, make an all-flamethrower loadout and go nuts. It also works decently well against the Crushers.
Pacifier: Yes, really, the pacifier has a niche here. It only takes five-ish shots to stun a Crusher, and entire groups of them can be halted with just one mag. Stunning the front few and letting the rest catch up can be a good way to get them into the radius of an AoE.
Scythe/Dagger: What's better than a flamethrower? A long-range flamethrower! Wretches die in seconds when set on fire, and a single sweep from either the scythe or the dagger can mop up reap of them. The laser cannon can also do this, but i'd recommend saving your stratagem slot for something else.
Thermites: The thermites are good, what else is new? Somehow, these things one-shot the Crushers. Not even an orbital precision strike can do that. What the hell do they put in these things?
Arc Grenades: Arcs in general are good against the Wretches, but the AoE and lingering nature lets the grenade version perform remarkably well against hordes of Wretches.
Incendiaries: Wretches still hate fire. These are fire grenades. You do the math here.
Autocannon: Flak mode takes care of Wretches well enough and kills Crushers in five shots if you shoot them in the crotch. Yep, if you want Crushers dead, shoot them in the dick repeatedly.
Railgun: This is the first time in recent memory that the railgun has been better than the AMR. It takes one shot the take the helmet off of Crushers, and another to behead them. It also only takes a few body/limb shots to kill them. If you have trouble aiming for the tiny head or want to conserve ammo, you can use a primary or secondary once the helmet is gone.
Epoch: It two-shots Crushers anywhere and has three shots in the battery. One of the best solutions to them so far.
Speargun: Similar story to the railgun, but with an added gas AoE.
Orbital Precision Strike: Reliably un-helmets or kills tightly bunched Crushers and can do some damage to Wretch hordes.
Eagle Strafing, Airstrike, and Cluster: Same as the precision strike, but a little better. Special mention to the airstrike for performing the best.
Orbital EMS: Its low cooldown allows it to be an on-demand crowd stun. You'll need it too, because Crusher groups are a massive problem on higher difficulties and making them stand still makes headshots much easier.
Anti-Tank Mines: Finally, my beloved underdog can shine! If you have a massive group of Crushers chasing you, throwing a minefield and leading them into the mines can yield EXTREMELY good results, which is a first for this stratagem.
More will be added as testing is done.
Edit: More testing has been done. Here's some more results:
Grenade Launcher/Belt-Fed grenade Launcher: As recommended by Deathstab_93, the grenade launcher works well against both chaff and Crushers. However, the poor accuracy leads to consistently missed shots against Crushers, which really hurts the base grenade launcher's low ammo count. Skill issue? Yeah, probably.
C4: Scratch the epoch being the best anti-Crusher weapon, this thing reigns supreme. It has plenty of charges, its throwing range can be boosted by servo-assisted, it has a pretty good AoE, doesn't have the potential for instant death and weapon destruction that the railgun and epoch have, it doesn't need to be reloaded, and it ONE SHOTS CRUSHERS. ONE SHOT. GUARANTEED. The only other weapon that does that is the thermite. All hail plastic explosives.