hmmm number wise the most optimal choice to prevent the enemy is indeed trench warfare, with the lines breach it would indeed be dangerous to fight such large number within a trench since their outnumbered 10 to 1, but then again they have the flood gates, if by chance its connected to the trench lines then we might just see a reenactment as close as it could be of the strategies used on the battle of Xia pi where they flooded the fort or in this case the trench no matter how shallow crossing water filled trenches would greatly hinder an army, crossing over it would also mean the soldiers would get sopping wet and that water will drop on ther otherwise dry undugged lands possibly turning it into mud which is a high possibility considering this more 10 men crossing those water filles trenches even half of those number would turn dry land into muddy paths, not to mention the possibility of washing away the enemy already on the trenches when the floodgates open which would decrease the number of enemies, time it right and attack the enemy from affar when they are greatly hindered on the water filled trenches and mud lands would be a devastating turn of events, of course not to forget the prototype landmines we've seen earlier, honestly as long as they could find a way to get rid of the enemy magician they will win that war, if it were me I'll get rid of those magician via those tribuchet but timing the bomb right would be the deciding factor there as well as aiming correctly, OF COURSE THIS IS ALL HIGHLY HYPOTHETICAL