They tend to get targeted when pushing siege engines (sometimes even dived!), so be sure to have enough ranged resistance! (Yes, you could put those higher tier doctrines on them, but for a grey unit? nah, just tier 1 or tier 2 is all you need in order for the doctrine to have its effect on the marts. Yes, they die to one scorpio shot, a lot of hp doctrines won't matter as they'd be barely under the scorpios total dps threshold. You'd have to be a Longsword class with heal in order to make them be able to take a final shot.)