If you are really into it, you might consider getting the guides. After playing WC3 a couple times through, I got the guide. I added my own notes to each mission and got so I could play through each one on nightmare with guns only. The tip to "aim better" was actually a good one if you take it not to mean the long shots, but to mean manuver yourself (or trick your opponent) to get close and make your shots count.
WC4 is alot tougher. There are more than a few missions that seem unbeatable on nightmare (most notably the dead zone mission after the mercenary pilot is picked up for interogation - if you DONT buy that same guy a drink at the games beginning - so the mission is like twice as long...I have never heard of ANYONE beating that one on nightmare). I never beat 'em all, but I did find a "gimmick" way to beat a few of the "unbeatable" missions using the leech - there are a couple of occasions where if you leech a craft, waves of fighters that would ordinarily appear no longer do (and interestingly enough WILL appear if you kill the leeched craft instead of autopiloting out to complete the mission). Try this on the mission where the Bearcats defect to your side, and you will see what I mean. In WC4, you not only really NEED your missiles, you have to make each one count in the tough missions because the AI is so much more challenging. It's my vote for best game ever made.