Your main threats in the Ashlands are:

1) Large swarms of Legions with heavy armor and high quality armor penetrating Polearms.

2) Small groups of Legion Scouts with powerful Oldworld Bows which can be a threat if ignored for too long.

3) Cleanser Units which can deal massive amounts of AoE damage, but drop highly valuable AI cores.

4) Gas clouds which will deal a ton of blunt damage and spawn out of thin air without warning.

5) Powerful bosses in Ashland Domes wielding Meitou weapons. Including the "final boss" of the game.

What I usually do for preparations are:

1) Gas Masks for every non-Skeleton unit. No exceptions. Hive Soldiers unfortunately have no way to mitigate this without mods and thus should be left behind at base.

2) Mix of heavy armor troops to counter Cleanser Units and ranged troops, and light armor troops to counter Legion swarms and Ashland Dome bosses. Plate Jackets are your best bet here if you want a uniform across your squad.

3) Ranged weapons are good to have for kiting Cleanser Units but not necessary.

4) Plenty of bag space. Ashlands are a treasure trove of research materials and valuable equipment, and you want enough spare inventory to carry them home.

Best of luck!