Not sure what you mean about the left/right movement, could you give more detail?
I played it some more, and got a bit further this time. I guess what made me confused a bit about the horizontal movement is the lack of a moving ground -> I can only see that I'm moving compared to the monsters. But the monsters also move, and that makes it a bit hard for me to see whether I'm moving, or the monsters are.
Avoiding the monsters at the first level seems pretty much impossible for me. However, I also noticed that this does not appear to matter much since there's hardly any penalty for being hit. Personally I would prefer it if there were fewer monsters initially, and if being hit was a bit more consequential (e.g. something like insta-death or 3 hits = dead). My actions don't really feel meaningful in the beginning.
Having played it a bit more, I must say I like the over-the-top abilities; flinging around fireballs while flying and teleporting at will is fun