What I'm reading here is that if you prioritize NightSisters, then you can get more than halfway there on the first time you hit an NS squad, then when you've completed the sector, you only have to use 1 battle just to complete the feat.
If you only need to use one battle to complete a feat beyond the battles required for progression, I think the feat is completely reasonable.
Also, what's with worrying about extra battles at all? You're in Sector 4 already? There aren't even 20 battles there, and you get 120 energy/day, which is 8 free battles per day. The event lasts 2 weeks. So...you've got another 80 attempts to succeed in less than 20 battles, and then you're going to need one more for your feat, which will still be less than 20 battles.
Sounds like you don't have any worries at all, do you?