Ash is super annoying to deal with. Her damage output is surprisingly high for a tank, but only at close range(unless they have slug shot). pick her off from a distance if at all possible. if not then wait for backup, you Will need it.
If your Max HP is 2000 or less, a single shot from Lex will put you within execute range. With a headshot this goes up to 3000HP, and With Death Hastens these numbers become 2800 and 4200 respecticely.