Missions will definitely get you to at least one of the last levels, then you could switch guides but
you're far off at the moment (level 12 is quite low, I think you missed lots of missions).
Hope this helps:
This will help by telling you what missions you still need to do (while pressing J in FusionFall will
show you the missions you've completed):
"Sometimes you gotta run before you can walk." - Tony Stark, Iron Man.