Tactics for the Swamp (Fortress).

I read that the Swamp faction is the weakest. They only have three levels in their town. So, is there a chance to play with them against the Conflux, for example?...

The Swamp and Stronghold factions require experience from the player. The opinion that they are weak arose because many people play the game, but only a few play well. A couple of years ago, I thought the same thing until I came across this portal. Then I registered and am now slowly studying the materials, changing my play style. Yes, the Conflux is one of the strongest factions, but not because of the 5th-level guild, but because of the double army growth, good speed, and overpowered heroes. In essence, you can realistically get 200% bird bonuses on the second week, which means there will be no problems with development afterward, and with some heroes, you can even start conquering Utopia. At 160%, the picture doesn't change; the only difference is that the bird bonuses will definitely be there (in 95% of games).
In general, for everyone who is just starting to learn the game and wants "results" tomorrow, it is better not to choose the Swamp or Stronghold factions. Also, it will be easier for you to play at the beginning with a mage hero rather than a warrior. And it will be easier to clear neutral creatures with archers than with, for example, wyverns or basilisks.