When the heavy blockers advance to the top of the stairs take out one (my Karin can do it in two swings and has a noisy attack so it's easy to time) and then target the boss directly so your units will run through the gap before the respawning heavy can close things back up. I do things with just Karin as a strike while everyone else clears cannons, gotten used to who to point where when so that she helps with cannons but engages the heavy first so nobody "helpfully" takes out the other one and makes a mess. She's level 50 though and with her healing ability can hold her own.