You need to calculate each move so that you use everything available on the map before the Necromancer arrives.
Well, you definitely won't be able to use a lair of powerful creatures effectively; 36 monks will destroy at least 50-70% of your army, so I don't think a couple of powerful creatures will be of much use.
I used everything else that was on the map (I didn't try to swim, though), and the losses were minimal: a couple of dozen swordsmen, a dozen peasants, and a couple of archers.
