Yeah, they can be a bit of a pain in the proverbial ass. Keep in mind that slashers are helpless once you get to shoot them. Problem of course is getting close enough.
Posbile tactics per factory
vehicles
A combination of ravagers and scorchers
Cloakies:
a zues and some glaives or warriors.
Tanks
a reaper laughs at slashers.
Shieldbots
dirtbags, 1-2 thugs and bandits
or
thugs + outlaw (well, this works against everything :P )
Throw in the high HP units first, they'll act like a barrier, when they are about within half of the slashers range send the rest.
Main problem is that you must know if you can win the fight before you start it, otherwise you are likely to loose a lot and give away metal. Meaning the slashercount will be even bigger in the next fight. You will learn to judge this over time.
Lastly, keep the T-Bone maneuver in mind! Saktoth tried to explain it already. Dont attack a line of slashers head on. Always try to attack one end of the line and put half of the slashers "off-side" so they cant shoot.
(MS-paint rules!)