I have been trying to level up Marnid and Borcha for a while, just letting them go solo against a bunch of river pirates, with their awesome armour and weaponry. Marnid has a courser so he isn't able to run through a bunch of them like borcha on his charger would. That means if Marnid runs into a bunch of them, the courser will get stuck, get killed, and marnid will very slowly be worn down by the hatchets bouncing off his black armour while he's in stunlock. Meanwhile Borcha will smash straight through them wih his charger.
For a change i tried them out on sea raiders. Marnid ran into a bunch of them and with a flash of axes was instantly KO'd. Borcha did the same, and was also instantly KO'd.