This evening, I practiced kenjutsu* with** my shinken***. I cut the air 100 times. That air is dead.* (lit) kenjutsu exercises. I wasn't sure how to have "practice" without the word 「れんしゅう」.** で？*** sword
Isn't "kuuki" more like "breath" than "air"?
I think generally "with" should be で. Interestingly the form 真剣に, which uses the same kanji, is an idiom meaning "seriously" or "earnestly."I think it would be 剣術（けんじゅつ）を練習（れんしゅう）しました or 剣術の練習をしました. Not 100% sure though.As far as 空気（くうき） goes, I think it does mean air or atmosphere in the way that Marshdrifter intends. Breath would be 息（いき） I think.
I just noticed, but shouldn't that be 運動（うんどう）, not うんど?