Marshadow is a powerful Pokemon. He’s definitely not someone that you can afford to underestimate. Trust me, it just won’t go well. Still, Bass is the strongest being in all of media so you can cut him a bit of slack there. Even if he looks down on Marshadow to an extent he will still be able to pull out the win. You just can’t hope to stop a being of his caliber. One Darkness Overload attack and it’s all over. Bass wins.
Suggested by Sonic Little Mac is definitely a solid fighter, but so is Marshadow. Marshadow knows hand to hand combat like the back of his hand and also has many elemental attacks to give him the edge here. Little Mac will be able to dodge and parry for a while, but eventually Marshadow’s superior speed and power will prove to be the edge here. There just isn’t much that Little Mac will be able to do against this level of ability. Marshadow wins.