I don't feel that it's necessary to have finished the game (or even played D:OS) to play through at the tactician difficulty setting. You just need to be decent at these types of games, have a little patience, and be prepared to try out different strategies. The weaker (combat-wise) you build your characters, the more outside the box thinking you'll need in combat. Heck, I think you'll have more fun that way.
Honestly, there are so few options available (compared to everywhere else) when you reach the first fight, I think it's more difficult than some of the battles that follow it.
