Oh boy, just so soon, I can't play the game anymore because the scene on the bridges with the "soldiers" is unplayable even in easy mode! I got passed the first 3 bridges but the next one has so many soldiers firing so many fireballs at me that approach so far, that I always get killed after a few seconds and then .... I have to start all over again and again on the first bridge. :-(

Is there a way to finally get this scene passed on easy level?

This is a really hard game (but I played at hard from the beginning), still it's fair. The more you play the more you will learn about enemies weakness. You just need to try again and again until you know how the AI works.
it's not like, for example, Far Cry 1, where there is no way to beat the game if you don't use god mode.