I killed 13.8k infantry/21 guns at Train Station, 9.7k/15 at Distress call, 15.8k/44 at Bull Run, 5.1k at River Crossing and 9.1k at Crossroads. This resulted in 42-47k Pool, 42-47% training 15-20% armory going into Shiloh which is close to the same as the video.
I replayed the battle a few times and got a victory by putting in a bunch of artillery instead of infantry. I kept the same number of brigades but this resulted in the AI having 4k less infantry and 10 less guns to the first day. Not too huge of a difference but the thing is that instead of having their max brigade size be 5.2k it was a massively reduced 3.3k as a result I was able to contain their breakthroughs at the Landing and get a day2 win.
One of my attempts used about 1k skirmishers but that didnt seem to affect the enemy army size much
edit: reading some of the previous comments this may be related to me having relatively large brigade sizes. I haven't put many points into army org so my average infantry brigades are closer to 2k men instead of the ~1k in the previously discussed video