In my experience R.O.B. is really bad in time mode because he lacks the ability to quickly kill the enemy so he ends up never dying and racking tons of damage but having a horrible score. All that matters in timed FFA is your ability to kill quickly. Mr. Game & Watch is a good choice; just spam down smash and go after whoever is at the highest percentage. Marth is pretty amazing too; Counter keeps you safe, and smash spamming works well. Wolf is another top character in this mode; if you just spam fsmash and dsmash and laser with a little Reflector everyone will die and no one will hit you. Meta Knight is pretty good if you are good about saving dsmash, but he's not really optimal. The heavy and slow characters all suck because they get smacked around too easily and never get their attacks off because of how slow they are; don't pick them. Any character with even a hint of KO problems (Diddy Kong, Sonic, Sheik, whatever) sucks. Guys who are somewhat fast with high power who I didn't list (Zelda, Ness, Lucas, Pikachu, Wario, Olimar) are pretty good, but they aren't ideal. Spammers like Pit are good if you are really accurate at stealing kills but overall aren't really worth it.
If you are an Olimar main, you can go with him as he's fine. Just remember the fundamental rule of timed free for alls; pick on the weak players. Kill total is all that matters; just go after the weak players (or highly damaged players) and spam kill moves. Racking up damage isn't really that important; don't bother with it too much (if you don't rack up damage, someone else will for you). If you are so lucky as to get stock mode, ignore everything I said. Pick a good camping character (Olimar works again!) and just avoid conflict. Also, if you have it in you, politics can help. Making agreements to work with other players can help you make free for alls even more broken and degenerate, but they'll be broken and degenerate in the right way for you to win! Speaking of degenerate, these modes really are degenerate. If you have a smart sounding strategy, it's probably bad. Good strategies for free for alls usually have about as much depth as "spam X powerful move".