I haven't played Heroes 4 against another person, but against the computer, any tactic works... and it doesn't matter what you choose, although I prefer a defensive style, so harpies with retaliation and cyclops with ranged attack. Birds are cheaper to build... that's how it is.
P.S.: I didn't think that Heroes 4 is still so popular, maybe I'll dig up the disc and remember how these little figures move around:)