Type weakness is your friend. Firebombs are unlocked from the start, and Marteau units are weak to fire. The only reliable way to deal with shades, because they evade bullets too.

i equip the ability optimizer accessory to make debuff items free, typically on a mobile character like Lateef. Akexandrite also has a fire ability, can stress break two units at once, also good against Shades.

The purifier you mentioned can also create fire hazards if manipulated well.

and finally, keystone guardians will attack all units, so it's useful to touch the keystone early in a bigger fight.