Each of the two duelists eliminates 1 castle out of 6 possible (the non-host goes first). From the remaining 4 castles, the host chooses the first castle, and then the second player.
The point is that the host has an advantage:
1. The non-host eliminates a castle – this is normal.
2. The second player, after thinking, will come to the following conclusion. Since he chooses first from the remaining 5, he will not eliminate the faction that he needs and will most likely choose the strongest one, and therefore eliminate the next faction after the strong one from the 5.
As a result, the host has a choice of 5 factions, while the non-host has a choice of 3, and I think not the best ones.
P.S. Maybe just make it like in the World Championship. Eliminate down to 1 faction, or use a random selection with a restart.