Forward tilt, shield dropping, predicting tech rolls and landing your F-Smashes. You can beat anyone with that. Mortar sliding helps, but be careful with it, it can be punished severely.
Space yourself well against both Lucario and Marth. They have incredible range. Lucario's F-Smash is incredibly long, as is his grab range. If he force palms you at a low percent, you're probably get chaingrabbed, so be very careful there. Limit how far he can move and punish his range by grenade dropping close to him.
Marth doesn't have a chaingrab as far as I know against Snake but he can still throw into an F-Smash. Be careful of his grab range. Spacing is really important against Marth players as a sweet spotted move from Marth will deal a lot of knockback and damage and can kill at medium-high percents. Use your knee and forward tilt sparingly as he can use his counter against those. Marth has no projectiles, so abuse all of yours (Nikita and Nades).
Many Marth and Lucario players that I have played LOVE stalling on the edge. Try to do this quickly: shield drop two grenades VERY close to the edge, roll backwards twice and then fire a Nikita. Once the Nikita makes contact with the nades, all three explosives will explode and knock the foe off the edge.