PaladinNO: Just build towers. Towers are so busted in this game, it's almost not funny.
Or upgraded Defenders (so they throw their hammers) in the case of the Dwarves.
Don't know exactly which missions you're referring to, whether it's a specific map or the whole Shadow of The Phoenix expansion, but the whole SP1 series is really way too easy, even on Hard difficulty, once you know the tactics (especially the cheese ones).
Just scout the resources needed to buy mass towers and any natural funnels if you don't know the map already, activate the monument and build up your towers in a semi-circle around said funnel. I often got 80-100 towers whenever I'm farming like this.
Then leave the game running and go to sleep, and watch your resources be in the tens of thousands the next day (all resources replenish over time). Build your army and then continue to sleep as your mass army just walks all over everything on the map. ;)
By using towers and leaving the game overnight also gets you maximum XP for your avatar - I am currently playing through Breath of Winter, and I reached level 21 before I got to Fastholme (to get the Firebane) by playing this way.
If you're struggling with the Blade Nightmares on the last map (and rightfully so, those things are real heavy-hitters), build Elven Druids or Human Paladins - both units got a hidden magic ability against Undead and Blades: It doesn't say anywhere that they have these abilities, but they do.
Enemy spawns are also getting progressively stronger after a monument is activated, as already mentioned. but only until a certain point. After that point, every base often have less units than at the start of the map (because all have been sent into your towers, and the AI have just "given up" trying to breach your defenses).
Sure, you can also attack the enemy bases with only your avatar (takes F.O.R.E.V.E.R. to destroy the buildings as a mage though), but this takes away 80 % of the XP you'd otherwise get on the map, and all the gold you'd get from loot, so I've only done this once as a test (and I restarted that map).
As you may have read my previous post - it WAS a bug, solved anyway.