Getting high rank is worth it eventually as it unlocks new spells, summons and rare equipment. For the most part you'll get a bunch of spells and shit as regular rewards, but the high level stuff tends to come from A rank (or max being S? I can't remember) when doing it on Hard or Agito (which have different rewards). You can be like 10 levels under at least to be safe, but much lower if you have confidence in dodging and hitting killsights.
As for hitting killsights with various long range characters you have to keep in mind their projectiles travel at certain speeds. Some characters you have to predict the killsight appearance and have an attack already flying (the timing is odd because if you attack too fast the killsight won't appear). Which is why certain characters (King/Cater) are better as starters because you don't need to predict and get the timing right whereas Ace and Trey require a lot more precision. Keep in mind that Ace's back normal (floating charged cards) travel faster than his regular attacks as do his skills.
The only thing I remember being missable is in chapter 7 with a few cut scenes related to those characters who can potentially join class 0. I think the 1st playthrough is the only chance you can see them (they aren't major) but that only happens if you accept their requests on chapter 7 which on your first playthrough is insanity so they're easy to avoid. Ultimately nothing important since everything else can be done in NG+.
XI Wiki



