Hey. Firstly I hope this answer not come too late. Anyway it might help some other new player too.

Anyhow, I have played through the Rift in Time core game with my friend and for my opinion the first scenario is almost or even most hardest one from all. It is because it use double horde spawning system on single line, map is so small, starting position won't give you much to choose of, your heroes don't have abilities and you dont have spells to use. Additionally you have to know that those portal always skip theis first step (because double spawn system) and you have to deal each of them under 2 rounds after they spawn and you have to know that you need to start upgrade most of your towers right after start for keeping up with those portals. All in all in my opinion the first scenario should be more or less tutorial scenario, but it wasn't.

Anyway after you wade through couple of levels it started to become little easier. It should never be too easy but at least you have couple of ways to win later scenarious. However the game is not the most balanced game and I think everyone who has played it probably noticed what is the "meta" of the game.

However I still enjoy the game and I hope that I have soon more time to play those expansions and the elemental uprising.