The Gunsword isn't a bad weapon, just a poorly implemented one that could had have improvements patched in if Capcom cared any about LP2 at the time.

You're going to want Melee I + Soundless, Fire Power II, Fire Power I + Critical I or Defense II. Any of these will improve your chances of survival. You could even go with Head Start Plasma and another ability of your choice such as Invisible to help you out as well.

Unless you're planning on wrecking Vital Suits with it, do not use Gunsword SP. The extra swing is not worth it otherwise. You're going to get stuck in the additional animation frames more times then not, and that will get you killed more times than not. Not to mention that the bullets from the normal Gunsword have a bit more power to them.
I could be wrong in saying that's it's also more acurate than it's SP counterpart. It's been awhile since I worked with either of those weapons.

Sometimes it pays to shoot your way to advancing to your opponent. Unless you feel confidant enough to kill someone from behind, don't go for a lunging kill. Unlike Halo, your opponent can knock you out of a lunge just by shooting at you. Might as well hurt them on the way. Just make sure your going for headshots.

Pay attention to your lunge distance. You'll still get the lunge even at unsafe distances, as well as an experienced player can roll out of the way.

Do not mash the sword. That is a sure fire way to get yourself killed. I've played with and against many that have saved themselves some by rolling after the first swing.