I recently cranked out some N! AoB attempts and I'd say the bigger problem is the unbalanced difficulty between the 4 possible Phobos Entries.
The quoted one is the hardest, regardless of difficulty or challenge because of how open everything is and how close the two sergeants start. Also, this is the only one where you can play HNTR and still have to fight for the medpacks.
The second hardest is the really big castle-looking one. Again, it's very open. The difficulty is magnified on AoB where you can't fight the sergeants one at a time, unlike in the other two where you can normally fight the top one and sneak inside, leaving the bottom one alone.
The other two are quite similar in most aspects and are the easy two. Even on N! AoB, I can often do those two without taking a lot of damage, especially if the upper sergeant is nice to me, shooting me twice at long range and only once as I close in.
I would suggest changing the layout of both of the harder ones to give them more cover, and in the case of the hardest one to stretch it out a bit so you aren't fighting everything at once.
A quick example that I think would make the hardest one more similar to the easy two in difficulty:
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#............····..··························..h.·········...................#
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#..............··#>[\h·+···##·#######····+····h···#······.................@..#
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(Red: Sergeants that appear on UV+, Blue: Humans that appear on HMP+)
Quick Edit: While I'm at it, why is "Frontal" togglable in the first place?