I think M2K and Emblem Lord have some great info but I'm not sure if we should stop all other discussion because of it.
Doing so would kind of defeat the purpose of a forum IMO. There's always new strategies and different opinions that can be discussed when it comes to almost any aspect of the game.
Personally I seem to get most of my kills off the stage or edgeguarding. For big kill moves I find Fsmash useful, but many of Marth's moves have good knock back so almost any smash or aerial can kill at high percents (or at least send them flying far enough to kill off the stage). Dolphin slash out of the shield is supposedly great for killing at high percents but I haven't had much success with this so far. Something you might want to try, though.
For the spike, I always try to jump or double jump just before it to make sure I don't fall after I use it. If you hang on the ledge, you can let go and then quickly double jump, followed by the spike and then dolphin slash back on to the stage. I find this useful if the opponent is directly below the ledge planning to recover straight up.