I must admit I agree with you, this fight is annoying me to no end, because of how utterly vague the duty directive description is (to say nothing of the lore aspects of the whole scenario). Personally I think they could have just skipped having it as a fight and just had as a long cutscene and it would have ended up the same conclusion, but there you are.
Either way, I can't even get to
Zenos, the first time I actually took too long getting to... you know who... instead trying to save the random Resistance troops scattered around being attacked by Garleans, when all of a sudden Krile shouted "NO! We're too late! We couldn't save our friends!" and 'Duty Failed' was my only reward. Then I decided to skip ahead and rushed to where Lyse and Y'shtola were, only to have my butt immediately handed to me by Fordola and her fellow quisling thugs as I didn't have anyone to heal me as Alphinaud and the others were still stuck fighting random Garleans.
I presume there is a specific strategy to it but I haven't worked that out yet.... Either way, in regards to Zenos, I've heard others you basically cannot win against him - he's like Beatrix in FFIX in that the battle is unwinnable, so to proceed you basically just have to get his health down to a certain level in order for the duty to be considered complete (my boyfriend said he got him down to about... 75% before the duty will complete). But as I mentioned I haven't even got to that point of the duty yet so I personally cannot attest to that.