World Arena

Markel isn't that overpowered =) If you take Railag with the "Master of Initiative" skill, Markel will be running away from him)))
Regarding the map - strengthen the garrisons, add a mentor for each race, please =)

Railag becomes good only when he reaches at least level 15 (and it takes a week or two to level him up), and Markel, with strong undead garrisons near the starting castle, can open the Dungeon on the second day of the game (which I did) and, as a result, get a couple of free level-ups and an open path, which makes him much stronger than everyone else on this map - all that remains is to run around and kill enemies.

Here are the step-by-step screenshots: descending into the Dungeon (day 3), capturing all three enemy castles (days 6, 10, and 11), and the final cutscene - this time, I used another hero in parallel with Markel to capture the cities, and as a result, the game ended on day 14.

I don't think you can win so quickly with Railag (or with any other hero) on this map.