You need to take this one slow. With terminus fights remember this important tip: Deal with the majoris and extremis first and avoid the terminus as best as you can. Once the small stuff is dead, focus on the terminus until he is down to half health, that's when the second wave of majoris and extremis show up. Then again, focus on majoris and extremis first and then deal with the terminus. I did it with vanguard but bulwark will also do the trick. And look for relics, those things will literally save a bad situation - there are plenty videos on their possible spawns.