Actually the best strategy i found for late game is to make 2 vgood heroes and send one for suicide mission with armagedon. This actions cuts down number of opponents units to possible win scenario for your main hero.
Its not easy to perform and requires practice but its possible and works really well. I myself completed it only once (mostly coz lvling 2 heroes makes your turns soo long)
Effect was awesome but still i lost the game cos i couldnt keep up with killing his troops(he was inferno) and after few hit and runs i made a mistake and he killed me wqith just a few creatures he had left. Im saying this just to show that i coulnd win vs late infeno even after free 20 sp 5 luck geddoning (luck didnt trigger tho;/) before the fight so i dont c a way i could win in a "normal" fight against them.