There are no positiionals required on trash packs except the initial use of Twin Snakes, which, who cares, it's a dungeon, it's one use of twin Snakes you are going to miss.

As for bosses, you need to know what they are going to do. I believe Argath is the last boss of Rabanastre? If so, when he does special attacks, he always faces south, you need to remember the line between rear and flank and be straddling that, remembering you can go inside the hitbox to straddle these lines as well to make movement to the correct position easier, which is something I very rarely see people do. They always seem to want to go around the outside.

However, at the end of the day, you just have to accept you will miss some positionals, it is not the end of the world and it will not tank your DPS unless you do it constantly. So, just like BLM, fight knowledge is crucial.