I think it's time to tell you something,Theroy.I know some good lvl30 humans that can crash everybody who stays on their way.
Why humans are good?if i tell good reasons promise that you will really promote me in my human skills.
First,humans are the best casters.Blizzard have made them to use magic,thats the strong in humans.What spells?Starting from the heroes,Blizzard and Flamestrike some good spells with AOE which can kill and make you win.I notice that humans,like me use this BM and AM strat on a group of units where they cant escape and this can own.The spellbreakers-not so used but you are free to cast blizzard and flame on them and I've experieced breaker strat so i know that helps.A good combination is Mk,he's not so good for the strong human combo for which they are made but again,thunder clap can stun them and again make some severe dmg with bliz or flame.
Second the units:Ne may have better spells but not all-the slow,the most common slow+riflemen,a cunning strat.Invisibility,when you want to reveal or hide your hero in low hps from enemy ,you can find it useful.Poly,now i dont like that it can be dispelled but much better vs air and the best vs wyrms,chims.The priests have heal and all hood humans make 7+ priests.Dont wonder when you lose.I once lost with riflemen because i had few priests the other human had rifles and many priests.Dispell :works not only on skellies,summoned are also animate dead units as wells as BM(beast master) units.
The footmen are the worst,rifles are just 'born' to be with casters.The knights are more mobile than any tier3 melee.The pally has important +defense devotion aura or heal.I have repeated many times:holy light is better than potions so if you dont need so much another mage ,try with him,you will heal faster than the stupid potions.If you have potions though very well even better.
Not,the air is worse than some.Nice gryph dmg and hawk spell-the net (blind),but they are weak and can easily die.The strong part are casters.
Tanks or other sieges(i wont mention gyros) so so,tanks can level a base to the ground and are cunning vs gargs,hyppos,but not tier3 like wyrms...
From all that it seems various strats are possible but only one is combined well-casters 2 mages,brilliance aura for them,the ranged spells and others.