this might be a silly idea, but I watched some guys playing Firebrand the other day and they were getting rocked by Striders assist. Have you considered something like Doom's hidden missiles or Taskmaster's vertical arrows to cover you in the air and help you get in? I don't know how it would work, just thought about it earliar on.
Well, in my roughly 1 week experience I honestly don't have much trouble getting in with Firebrand, he's got great mix ups. I like horizontal arrows better for a nasty M hell's elevator crossup.
Hidden Missiles definitely works too, but I just can't grasp Doom combos at all. I've wanted to play doom since early in Vanilla, he's just outside of my abilities. Maybe some day...