Senteria,
interesting question, and although i'm hardly a seasoned player,i'll be the first to chime in.
i haven't done an assassination run in ages -- only did it once, in fact, and that was from sheer desperation in one of those AoW 1 tri-level scenarios (although i'm sorely tempted to try it now after hammering away at an AoW 2 campaign mission for a week).
i still do my campaign missions the old-fashioned way: capture a town, build it up bit by bit until i deem it safe to venture further (which all too often backfires), then look for another city to capture to deprive my enemy of the income and troop-building capabilities.
yes, i also beef up my heroes any way possible, giving them available enchantments, exploring ruins for the equipment, etc., but in the missions i've been playing lately, sending them forth to kill the leader doesn't seem like an option.
as far as troops, i've done it both ways -- scads of level one archers, shard-throwers, etc., to defend a town, or if i have the luxury, witches and priests can hold a town indefinitely, and when it's time to leave and fight on the ground, they're pretty handy there too.
as far as fighters i find it excruciating to wait 15-20 or more turns to wait for an Ice Dragon, Angel, Elemental or other high-level summoned creature, but it often proves to be worth the wait (but, oh, the misery when they get hit with Banish Summoned spell).
ultimately, i've found the most crucial factor in winning a campaign mission is simply learning what's where on the map, meaning the early game is often an exercise in frustration (someone could do a great public service by publishing campaign/scenario maps, but i suppose that would be cheating, eh?).
that what you had in mind? will keep an eye out for other responses, as i'm currently getting smacked around in the AoW 2 campaign, and i'll take all the advice i can get. cheers.