It's true that the earlier mastery levels are balanced to be quite easy. This is to ensure even players new to this kind of game have a shot at being Talented!
Even given that we think the game should be a bit more challenging from the start, so we'll be shipping some nerfs to some of the more broken strategies (looking at you Archer) in 0.10.
Endless mode is on our Steam Roadmap! So you can expect to see that in an future Steam Update.
Thanks for the feedback!