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.
10 times I died at the start. 2 additional archers, and the fact that you only have 2 characters at that point - leaves no room for mistakes.
Then it is a bit difficult up until lvl 4 or 5, but from there you have so many options there is always a way to win.
Point: Just push through the start even if it seems a bit hard, the fights further into the game are much more diverse then on vanilla.
