Might as well post those here, for people who are curious about what the fuck is going on behind the scenes especially on that one map.

https://www.youtube.com/watch?v=IMcAn6Z25AE
(Effect of attack/defense boosting items and the drums on off-screen ally performance)
https://www.youtube.com/watch?v=IRWjH0lMs8U
(testing raw ATK/DEF differences)

tl;dr
Having more attack than defense in DW3 makes your allies survive more.
Having more defense than attack makes our allies die faster
The fastest time possible for all your army to drop at Yiling in Very Hard is around 1 minute and 30 seconds, the longest time is over 40 minutes, although those are only achievable with really large differences not seen in normal gameplay.
If you equipped the tortoise amulet it was your fault all along.