Consider if you've levelled up enough units/carried enough gold over from previous scenarios.
This one is very hard to overemphasize. If you are having trouble with a scenario, go systematically through the list of types of units you can recruit, and see what each of them can become if it levels up a couple of times. Would any of those types of units help you beat the current scenario more easily? If so, that's your problem: go back and level up your units before you get to where you need them.
A good example of this is the Siege of Elensefar. Having trouble with that level, you could of course search the fora and read tons of discussion about it, but also you could use reasoning along these lines...
Hmm... Okay, I can recruit Elvish Fighters, let's see, those level up to Elvish Captain or Elvish Hero, so let's look at the descriptions for those... hey, that looks like it would be nice to have one or two of those. Hmm... what about these Elvish Archers... Oh, hey, Rangers have more hit points; maybe those would survive better against these grunts here. Maybe I should have levelled up an archer or three. Hmmm... What about these Shamen? They're basically worthless in combat... what do they level up to? Druid? Oh, hey, look, those have a "cures" ability, what's that? (click) Oh, say, that would sure be nice on this scenario with all these thrice-becursed Orcish Assasins poisoning my units all the time. Wow, maybe I should go back and replay the earlier scenes and get me some level-2 elves for this one (and maybe a couple of Knights, too).
Similar reasoning works at the Fords of Abez, when you find that your units can't move worth squat in the water. Hmmm... you look through the stats of the unit-types your units could level up to, and you discover that if you can get one of your Druids levelled up again to Shyde before you get there, it could then actually move over water quite nicely. So you check into higher-level mages... nope, no joy there, movement-wise. So you might have to not recall the mages for this scenario; bummer. Okay, but what about your other units... the Gryphon Riders move great over water, but they're so wimpy... say, what if a couple of those were levelled up... and so it goes.
By the time I finally finished the Heir to the Throne campaign (medium difficulty), I'd gone back a few scenarios for better units half a dozen times, and I ended up with half a dozen Dwarvish Lords and a couple of Thunderguards, a Great Mage, an Archmage, and a White Mage, two Shydes, a couple of Avengers and a Sharpshooter, a Champion and a Marshall, three Outriders, four Gryphon Masters, two Paladins, two Grand Knights and a Lancer, a couple of Rogues, and two Merman Warriors with Storm Tridents, and all the principle characters were at level 3 or higher. Even then, at times I could have put more levelled units to good use, especially more levelled mages. It also would have been very nice to have more Avengers and more levelled horsemen of one sort or another. The only unit on the above list that I would do differently if I went back is the Sharpshooter; if I had to do over again I'd make it another Avenger instead, because those extra hitpoints are terribly useful.
That list is, of course, specific to Heir to the Throne, because it depends heavily on what kinds of units you can recruit and what sorts of terrain and enemies they're facing, but the idea should be applicable generally: experienced units are worth more than gold. Don't kill the enemy leader yet if by waiting another couple of turns you can mop up three or four extra kills for your units. Have you been trying to finish as soon as possible? Go back, and finish each scenario with the best *units* possible. Better units will earn that gold back for you, by making quicker work of your enemies.