I wonder if Sir Hugh would like to revise his strategy for this mission (both part 1 and 2).
The strategy of using 50-75 fire ballistae simply is not effective since the release of patch 1.4. Fire ballistae no longer target the enemy automatically, they must be manually controlled. That's nearly impossible with any more than 4-6 ballistae unless you like to pause the game, manually select each ballistae and each target, then unpause the game, on and on, etc.
I've enjoyed his other walk through's though, and his defence strategy for the start of mission 11 is still valid. Thanks!