To compensate for Odachi's relatively slow cast speed, you usually don't engage right at the mob's face, but keep a distant head start then start casting, of course after you gain aggro.
The head start distance depends on the skill you're using, but for the slowest casting skill of Odachi, Ougi - Crimson Flame Blast Smash, 10m is a good head start. I'll probably also include the mechanics of skills later on and how to use them for hit and runs.