Whatever happens, our ambush battleplan should always focus on
A) Holding the enemy hordes at bay via prepared positions and natural obstacles.
B) Focusing our siege weapons & flamethrowers onto the enemy abominations.
C) Killing Sombra by circumventing the slave soldier formations through/during the ambush.
The enemy will fight to the last. They cannot be effected by morale and apparently they are immune to the effects of cold.
As such, if we focus on breaking through them to get to Sombra,
we would need to kill every last slave on the battlefield, with Sombra possibly deciding to pull back if things end up going against his plans while leaving most of the slaves to a unending holding action.
The abominations are an issue in that they are units that can break our ranks. We must not let them to break our positions or we will lose cohesion and end uå trampled by the slave legion.
But Sombra is the important one here. Sombra is keeping the mind control up and is also likely to be one directing the monsters. If he bites the dust (or becomes temporarily banished, depending on the results of his defeat) all surviving slaves will be released while the beasts will likely rampage without leadership.
They are also the most overpowered individual there is on the battlefield. If we are doing any explosive surprises, the plan should be to hit as close to him as possible. I doubt we could take them out in one shot (the whole carriage is likely full of shielding spells), but such shields take power: power that Sombra wouldn't be able to use against us as his reserves recover from any hit.
So if possible, blowing up the mines under Sombra or in the highest abomination concentration as possible is paramount.
The yak chargers are made for charging, but they can also hold a line. The best use for them would be to slam into the enemy just after/during our initial ambush is triggered and then have them retreat back to our lines to strengthen defensive formations. Alternatively any yak elite warriors (like their chieftain bodyguards and the like) should team up with either two or all of our knight orders in hopes of flanking to Sombra when he least expects it - of course if we manage to trigger the ambush when our cannons are in range to volley onto the carriage, it would help us immensely.
EDIT:
This is not a winniable situation. We are not going to defeat this army, and the situations is more then likely going to end in the cannon events. IE Dues Ex Harmony.
-_-
Says our glorious unannounced prophet?