IMO Mash is very skippable.
Her defense isn’t great considering she doesn’t get the 50% damage reduction Rulers get and her offense is pathetic. Her targetable invincibility is really her only good feature and you can get the equivalent or better from Merlin, David, Skadi, Jeanne.
If Mash had the same cost as other 4 stars I would never use her at all because she simply does not bring enough to the table and realistically no one else would either for the same reason. Georgios is a better taunter and Leonidas would cost less for a 1 turn taunter.
For farming you need high AoE damage and for boss killing you need high single target damage.
From my experience you are better off relying on supports for AoE damage dealing for farming since that gives you a good AoE DD for each class. This will allow you to invest more of your limited resources into developing buffers who will be useful for farming and boss killing.