Nothing to intricate something like what above where we have the basic idea of what we want to do and quite frankly in this case ripping off 300 is probably one of the better options to go with since it takes away their biggest advantage over us which is numbers, and hey they took away our flight lets take away their numbers make it a little more fair.
I was thinking, if need be, we set up two 'areas', one at the base of the pass and another at the other end with the Black Pikes, and possible the chargers, between the two. The first we'll have our troops with Ballistas as artillery support which go for the magic abominations, Sombre and the 'docile,' Umbra.
When the numbers of Sombre's troops prove to weigh down our people we signal the troops using the Bird Call communications to fall back while Diamond Dogs collapse the 'floor' before retreating as well.
We regroup at the Midway point and have the Blackpikes take center, the yak Chargers begin launching raids and resting behind them, and have the Cannons be on the flank, behind the Blackpikes on makeshift dirt platforms. Again, Diamond Dogs will be invaluable because they have an easier time manipulating dirt to get an artificial ridge made. In the pass, the cannons only have one direction that they need to aim down. In the enclosed space, the charging yaks, and what appears to be a lack of spearmen amoung Sombre's Force, will be like an avalanche, a force of nature against the army. If Midpoint fails as well then we have the cannon crews sabotage their stores of Black powder which should cause an actual avalanche.
Finally, at the top of the pass we have the berserkers and the Flamers set up, both the stationery mercenary ones and the Kingdom's newly made flamethrowers. We make, what should hopefully be our victory over Sombre, a final battle against his bleedout army.