You only need to do one side of the map since the goal is to just kill the boss and most of the units won't move unless you approach them. I had my strongest units in the middle and the rest in the right. I went middle and up the right stairs with my mid unit and slowly had my right units progressing through that gauntlet. Having any flying units you got basically working double duty between the middle and right goes a long way too. Especially if you can pick off the one bolting unit up the stairs early.
The pegasus knight reinforcements are rough. All I can say is basically prepare your unit to be able to one shot them, otherwise they can probably gang up on and destroy your weakest ones on their third turn when they catch up with you.
