I always use 1 Champion + 7 Rogues to face 4 Heavy units + 4 Artillery units.
This is HMA, where the trebuchet reigns supreme. There are several of these 4 heavy/4 artillery setups, sometimes in both waves, in the end of HMA GE, where the DA combat bonuses ramp up hard. Normally, I don't even take damage against these, although I did take damage in the first wave (also 4 heavy/4 artillery) because I was rushing to try to finish, and didn't properly handle a situation with slow terrain (usually a benefit!) where the DA trebuchet hits left rogues on the bottom and the heavies were coming in on the top, and I couldn't quite reach a second heavy with a rogue.
On the first turn, the enemy trebuchets will convert 4 rogues, and together the six trebuchets you now have must take out the first two enemy trebuchets, as well as either the third, or one hit to each of the third and fourth. On the second turn, your first two trebuchets will finish off the remainder of the enemy trebuchets (they will get no more than the one hit they got). The other four will hit each of the four heavies, and if they are close enough (depends on terrain), two will get a rogue up the tailpipe, next turn if not this turn. Third turn, the heavies convert the rogues, and you now have 8 trebuchets to finish off 2 damaged heavies, which will be an exercise left for the class.