I've just updated the wiki with a fight mechanics note on this. You need to alternate between the two. I've done both melee and ranged and found melee to be the easier probably due to my high skill level in heavy armor. If you have shock resistance items, gear up. And I'm usually about 50lbs heavier after the fight for all the poultry I've eaten.
If you are doing melee, try to round both characters up so that you only have to pivot to fight the next opponent. If you are ranged, utilize the ledge with the barrels in the back of the room - Alyx doesn't appear to climb ladders like Devra does and the barrels will provide cover from the shock.
The fight is hardest at the beginning. After awhile, it only takes about 3 hits before they go to knee. Fight should start wrapping up around 20 knee bends each. A notice should appear in the top left corner indicating their shield levels. I've only noticed it at 25, 50 & 75%. But then again, the battle gets heated and I may have missed other percentages. Once you get down the pattern, fight shouldn't take that long, <10 minutes melee, a bit more for ranged.
Mendelus should become active only after one of the opponents goes down.
I've noticed that Devra will sheath her battleaxe on several instances and approaches you in battle stance sans weapon. Sometimes she will re-equip, sometimes not. This tends to happen when I melee more than when I range.
Hope this helps someone.