I understand how you feel during multiplayer battles.
I found a way to handle free-for-alls that works for me, personally. This is how I do it. I try to stay out of the fray, and usually stand on the edge of the stage. I let everyone battle each other. If someone approaches me, I grab them and throw them back at the other players. If several people approach me, I spam forward smashes and just try my best to stay out of the fray. I usually do NOT throw Pikmin at the other players, as it annoys the heck out of everyone and draws attention to Olimar. The key is to let everyone fight each other. Once I see people reach high percentages, I run in and up-smash them. Essentially, I am sniping them with Olimar.
The reason why I don't just let other players kill each other is because usually, in a free-for-all, people take A LONG TIME to kill each other. They just keep smacking each other up, and diminishing returns makes it difficult for anyone to land a killing blow. So, in order to speed up the process and ending the match more quickly, I do the killing. I DON'T do the fighting.
When players start getting eliminated, and it's down to just 1 or 2 other players total, that's when I start engaging in actual fighting. I use my normal strategy. I treat the battle like it's a 1 vs 1 match.