I used to main Ivysaur, so I think I have some worthy input here. Ivysaur outranges Ike, but our mobility is far superior. Your objective in neutral is to stay mobile and use fake outs (QD to WD, dash dance, WD) to bait out Ivysaur's far reaching but slow moves such as Fair, grab, and Dtilt, then proceed to punish with something like a QD grab. Razor Leaf is a great projectile, but it has a very slow rate of fire, so you don't really need to worry about it shutting you down hard. Try to stay at that medium range so you can threaten/punish Ivysaur if she attempts a Razor Leaf but you won't get hit by a sudden long range move like Dtilt.
Ivysaur has a good weight and fall speed for Ike to combo. BnB stuff like Fthrow/Bthrow to Fair/Bair work well, and at earlier percents you should be using Uthrow and Nair to juggle like always.
Now for the toughest part of the matchup. Recovering versus an Ivysaur as Ike is extremely difficult. Ivysaur stomps all over projected recoveries, and QD is an extremely projected recovery. Try to mix things up as much as you can. Whenever possible, go low for an Aether instead of recovering with QD. If you're sent far and high, instead of going for the ledge, try to go high with QD. Just avoid that Bair!!
On the other hand, with 3.5 nerfs, edgeguarding Ivysaur should theoretically be easy (although I haven't vs'd an Ivysaur yet in 3.5). Think of it as the same as edgeguarding Falcon. Just get on that ledge, and wait for Ivysaur to tether it. Once she hops up, get up yourself and smack her with Dsmash. Repeat.
This went longer than I intended, but it should help. Good luck!
