Oh, I discovered another gross cheat for C4 planting today. This one is usable for 1v1 too (if items are on.) Do you ever find yourself in the situation where both you and another player spot an item at the same time and you both race to get it ... but your opponent got there JUST before you did? Well, if that particular item happens to be an Assist Trophy, you're in luck. Whenever anyone grabs an assist trophy, they always spam "A" to crack that baby open as fast as possible, because one, you can't use any "A" attacks while holding it, two, you don't want it knocked out of your hands, and three, Assist Trophies are amazing. ATs are basically the new Pokeballs. Anyway, your opponent grabbed the AT just a split second before you did. What do you do? Safely plant C4 on him. That's right, I said safely: The time it takes for his character to crack open that trophy is JUST enough time for you to plant your evil and run away. During that entire trophy startup time, your opponent has no way of attacking you or avoiding your plant. ATs aren't like Pokeballs where it can be thrown at an enemy, and your opponent is completely vulnerable while he is in the AT startup animation. Plus, you're probably not going to be liking whatever comes out of that trophy anyway, so get the hell away from there. So you lost the trophy, but make your opponent pay a price for getting it first.
There was a whole series of mindgames going on today in our crew over Assist Trophies because of my evil Snake tactics, LOL. People would be hesitant to pick up ATs whenever I was near, because they knew I'd run right in to stick them. But at the same time, they didn't want me getting the trophy either. We'd have whole BATTLES over ATs, sometimes multiple stocks, and it wasn't until the trophy started disappearing that people would force themselves to make a move, LOL.
I'd imagine that this would work with any other items that require some sort of startup time and that an opponent would want to use right away ... I can't really think of any others besides ATs off the top of my head, but I'm sure there are probably a few other items you can use this trick with. ATs are just the most ideal because of the perfect window of vulnerability it puts your opponent in.