Aha. Squirtle's hard to use in lag, too. But yeah, he's my worst.
Enochuout pretty much said everything. Ivysaur has a really easy time of it escaping from the chain grab for some reason, and it was consistent. I managed to get out with Charizard and Squirtle like once or twice because of lag, but Ivysaur got out like six or seven times. All I did was DI up and attempt to jump. Either Falco needs to be REALLY strict with the timing versus Ivysaur, or Ivysaur can get out after maybe two or three.
Ivysaur has the potential, it's just hard to get Falco in a position that lets you get the Fsmash off. Like eGale said on the Pokemon Trainer boards, Usmash can help as a prediction piece versus Phantasm.
Early edge-guarding attempts with Charizard were unsuccesful and Phantasm really does make it risky. Either set yourself for a Bair if he tries to recover on the stage, or wait him out. If you land Flamethrower, you can expect Reflector after it, so as you cancel the attack you should probably hold shield. If you shield the Reflector it leaves Falco wide open for a grab. But like he said, Falco's really suseceptible to Flamethrower as an edgeguard, similar to Link.
As far as I can tell about the Phantasm, it's open a tad in the middle (I managed one spike) and it's definately very open near the end of the attack.
In general, Ivysaur's Bair seemed pretty effective to keep Falco off, and Falco can get gimped by tether-hogging if you do it right.
I'd play Squirtle and Charizard center stage for this fight to avoid chain to spike.