Got this in one try, final time was 36 minutes and some change.
Play on NEMESIS difficulty for the multiplier.
First take the outposts, I went Northeast, then counter-clockwise. Upgraded reduced mercenary cost 3x first. Assign the cheapest possible mercenaries to each outpost you take, doesn't matter as long as there is a merc there it won't get re-taken.
As you take the outposts upgrade so that you can have 3 Bodyguards, they are Captain killing machines. After bodyguard upgrades I just focused on Health Elixirs and then the upgrades for Serka that let you blow open all the gates, then Bolts.
Once you have all the Outposts you can weaken the fort by killing Warchiefs or just assault it. With 3 Legendary Bodyguards all I did was kill 1 Warchief and then started the quest to assault, and won.
Combat tips: I just grapple and glide around and stealth attack the Captains/Warchiefs, most can be killed in 1-3 stealth hits. Becomes even easier once you have a bodyguard or 3. I don't engage in direct combat much at all, if any. The final "boss" of the Fortress is kind of a joke....use one execute on him and he is dazed for the whole fight and you can just punch his face off basically. Finished with a time of 36 minutes and over 400K points, giving you lots of wiggle room.