I am really glad to have sparked such interest and calculations... I am almost tempted to actually draw a tournament table and see what comes out
I imagined from the start, as I said in the first post, that it likely would be a rock-scissor-paper issue between archdemon, black dragon and ancient ent, so that means maybe a round robin between all lvl 5 units would be better to give a "winner" than playoffs.
Another couple of thoughts:
1) I said that if the tournament was held at night the troll would be unbeatable. Obviously he would not, however, be
undrawable, since many creatures can outrun him and would never get caught, even a tiny forest fairy for example;
2) what would happen if we threw in also the ancient phoenix and even the lvl 5 book of evil? The book is easily dispatched, but what is the intrinsic strength of it's spells without an "Amelie" behind it? Anyway I suppose it still would be no match for any lvl 5 and most lvl 4 units. But the ancient phoenix? In the campaign it's a more or less useless spell, but in a one-on-one contest I think it would rank quite high...