I think the idea itself is fine, but implemented extremely poorly. One of the worst offenders is round 2 (I believe) on Darkstar where the Prometheans spawn in right next to the location you have to defend and immediately rush inside. Meanwhile, every player is across the map getting new weapons. Just one example of how Firefight was barely an afterthought thrown in just to say H5 had it.
Yes that level is hard, if you play a good strategy, always that first round finish go to the base where you need to defend (even if you dont know what's going to happen in level 2) that map have 4 differents level 2 rounds and the 4 are close to the base.
I always use there level 3 plasma caster or level 2 splinter turrent, walk close to the stairs and got the most kills. The only guy that can beat my weapon strategy is someone with ghost but I find boring just winning using vehicles, I like some challenge.
If you're lucky and in first round darkstar they put the two hunters, there is a moment when two elites arrive and they have plasma caster😛, kill them from long range (because they kill you one shot in legendary) and now you can kill easy the hunters and you have like 15 bullets more for defend the base without spend your level 3 req and can pull an overshield level 3 🍻