It's not impossible I have them. The key to getting the really high score is getting your multipliers way up and then going into focus mode and killing the doom trolls. This is a lot easier in the later levels once you get 40 Below, but in the early levels when your warrior choices are limited it gets trickier. Another thing you have to do is build up all of you multipliers (carnage, brutal, and frenzy). This means not getting hit or especially catching on fire or falling off cliffs.
Also make sure when you are killing trolls that you are not just racking up the carnage multipliers. Your score for each kill is troll points* carnages*brutals*frenzies so without all three you will never get the A ranks (you can see the difference below). For a rough example if a troll is worth 10 points and you have the following multipliers scores your score for one kill will be as follows:
Troll points * Carnage * Brutal * Frenzy = Total points for that troll
10 * 50 * 1 * 1 = 500
10 * 50 * 40 * 40 = 800,000
There are a lot of ways to get all the multipliers up, for example in the first level I used Chilla to though everyone on my back then hurl them over the ledge, I think you get something like 3 carnages, 3 brutals, and 2 frenzies for every 3 trolls.
Another thing to look out for is multiple troll spawns. Every level has at least one but many people miss them because they don’t start spawning right away. You need these extra trolls to get your score high enough to get the A ranks. On Thorn’s castle there is one after you kill the three white trolls, on the Forgotten Forest there’s one after you rescue the first little shrub guy and then one after the second set of white trolls.
Hope this helps let me know if you need any help.