I feel like I’m going around in circles, is it just a case of get good?

Basically the title, I’ve been playing for a few hours now with my partner and the randoms in matchmaking and been grinding against Battle for Tertium mission 1 and just have not been able to beat the captain that comes out at the end. Is it just a case of repeating the mission until we defeat them or am I missing something? Because everything else is locked until moving forward in the campaign.