Is the difficulty too high? After winning twice, the level of the monster has been upgraded to level 5 or even level 6
(all the contents of a China player's message are translated).
Its quite random i would say.
Basically monsters mostly spawns on stage 1 or 2, they will fight other races and grow in power, so over time they snowball and grow to stages 5+ by beating other monsters. So for now if you feel that monsters becoming to strong i suggest to regenerate map, you can do this by waiting for end of week or going to tower in city (North building) and talk to Lorena over here.