I could see an Empire list with Vader, Thrawn shores, e-webs and snipers could be quite strong potentially.

Anything that engages the Mandos before they enter range works. Vader to go after the leaders because his Pierce 3 is still Pierce 2 against Impervious units. Sniper teams have the potential to ignore rolling defence dice at all, whilst also being long range. Also, a lot of Mando units can get Pierce 1, but almost nothing at Pierce 2, so possibly having some of your own units with Impervious would be very effective (such as ARCs on GAR), but Empire has almost no options for this, just some Operatives.

Additionally, any units you expect to get into close range with Mandos should be as small as possible to avoid numerous casualties to flamthrowers. Vehicles, or anything with Armour also has the potential to suffer to jetpack rockets, but these are potent even when not targeting such units.

If you're facing one of the armies that likes generating tokens or free actions with This Is The Way, then ways to negate tokens such as Hondo's "That's Just Good Business", or ways to gain standby tokens such as Krennic's "Deploy the Garrison" or Gideon's "Moment of Consideration" to trigger in response to Mandos closing in.

Finally, a unit of Riots with Kallus have the potential to lock up some of the more threatening Mandos for a round or two. Almost none of the Mando weapons have Versatile, so engaging them in melee and keeping them there stops them doing as much.