The Force does help in combat. In fact the force is what makes a Jedi so remarkable and dangerous when in battle. The only reason none of those Jedi were able to predict their own doom was because of one single thing: the Dark Side clouded their vision. It has been stated many times during the prequels. In the case of the Sith they were unable to defend themselves for a matter of reasons: Maul did not believe Obi-Wan was a threat when he was hanging for his life. His overconfidence was his end. The same happened to Anakin when Obi-Wan made BBQ slices out of him; his overconfidence and the fact he was blind with rage. Luke during ESB was only an apprentice, hell not even an apprentice. Vader did see it coming, but he simply could not defend himself against Luke's onslaught. I could waste my time and explain every situation but I don't have to, nor do I want to.
There is no fact suggesting Sephiroth is more powerful than Kreia. You're just making that up to make yourself feel good.
Another funny thing is you're mentioning Yoda not stopping Palpatine etc, but need I remind you Palpatine is a Dark Side master who is an equal, if not more powerful than Yoda himself.
In Sephiroth's case he is not a master of the force, nor does he even know about it.