As stated before, Pikmin cannot absorb Razor Leaves. They go through targets. Dodging this attack is your best bet against Ivysaur. To be honest, you outrange Ivysaur in the grab department by a small bit. Also remember, Olimar's grab starts the second the Pikmn starts walking away from him. If you're point blank with Ivyaur and attempt a grab, I would believe that the Pikmin would just snatch Ivysaur back up. Olimar has one of the most deceptive grabs in the game, so use that to your advantage. Also, dsmash works just as instantly (if not moreso) than the grab. When in doubt, try the dsmash.
Other projectiles, such as Nikitas and Flamethrowers, use Red Pikmin Fsmashes or Pikmin Throws. Purples work when you're short on Reds in most cases.
Against chargeshots and Thunder Jolts, yellows obviously do the job. Fsmash is more pinpoint against these attacks. Pikmin throw usually lobs the pikmin over the attacks. Not sure the element of Aura spheres, but I assume they're electric. If not, hurl a Purple at those.
That's just to stop the bigger projectiles. For smaller projectiles, such as arrows, ROB lasers, and fireballs, I'd probably just learn to navigate around them to be honest. Remember that Olimar's grab range is more than you'd expect. Don't be afraid to use that when you're up close.