I've found that getting those 1000 kos within the time limit comes down to picking the right battles, like the xu province fight or that intense xiapi defense in chapter 3. The enemies there bunch up really nicely, and you'll want a weapon that can take advantage of that. From my experience, the spear works great with moves like rising dragon vortex, or if you prefer something different, the wheels can tear through crowds with blazing suns.

Something that's really helpful to know - war god rage mode is your best friend here. Hit l3+r3 when you've got a good group of enemies around you, and you'll be practically unstoppable while dealing massive damage. I usually camp near the reinforcement gates at xiapi's west wall, since the enemies just keep coming and you can use the high ground to your advantage.

Your guard squad can be super useful too if you use charge tactics to funnel enemies your way, and don't forget about cavalry shouts to weaken them up first. The gauntlets are actually pretty decent for taking out officers quickly which helps keep the enemy spawns flowing nicely. Just remember the timer stops during cutscenes and duels, so you've got a bit of breathing room there to plan your next move. Honestly, once you get the hang of positioning and timing your war god rage right, hitting that 1000 mark within the 10-minute limit becomes pretty manageable even if it seems overwhelming at first.