Proactive wall of text:
How Phantasm works: It can be canceled as you should know, though for offensive purposes this is rarely done. The first half of the Phantasm itself (the teleport) consists of invincibility frames, don't try to interrupt Falco within about his ftilt range, because he can't be damage at that point. Of course he has no invincibility during the startup frames, feel free to nag him there.
Phantasm is most often used when Falco is somewhat in control, this usually means he is fully capable of spamming SHDL without any fear of his opponent bursting his way through it. When he does feel threatened, however, the logical response is to Phantasm away. Sometimes this is done when Falco feels that his opponent is getting impatient and will try to just shield approach and grab through the lasers, shielding in front of Falco just outside of grab range usually means a Phantasm.
Of course higher-level Falco players will know that you know this, and will mix it up carefully enough. The deal is pretty much to either bait a Phantasm while not getting hit by lasers (which is easy enough for Wario to do), or not to allow Falco to get to it.
Option 1 takes more patience but may pay off, a predicted Phantasm means that Wario can space away to where the Phantasm will finish, powershield it and punish with nair/bair/dair/fsmash/etc. Whenever you expect a Phantasm but aren't sure whether or not you can space all the way back (or you just find the chance to be rather high, yet not guaranteed ie 20-50%) you can simply nair or dair way out of Falco's range. If Falco decides to Phantasm he will eat a weak hit of the nair or the last hits of dair, and he will be in a rather silly situation, one in Wario's advantage. When in an on-stage situation it is not probably you can catch Falco in the startup frames, as he'll usually be too far away.
Option 2 is playing aggressively and keeping control of the match, and not practical unless you can deal with option 1 as well.
The only other practical situation for Falco to Phantasm is when he's offstage or on the ledge and wants to get back to where his character design does, in fact, work (for the slow people among us: that's onstage). Aside of the standard ledge options, Falco is blessed with Phantasm. If you expect a Phantasm you can beat it (non-canceled) with random dair/nair, or simply fairing Falco in the startup frames and practically instantly gimping him (since he has lost his DJ). Don't try to just shield near the edge, that doesn't quite work since his ledge roll is fast enough to just get away with it. You should space just outside his ledge attack range, stay shorthopped and always be wary of Phantasm. It makes ledgeguarding a bit more difficult, but then again it's very much capable of being dealt with. Beware of Phantasm cancels to catch you off-guard.
Oh if for some reason you find yourself on a stage like frigate, in which you can sometimes get a guaranteed non-ledge-sweetspotting Phantasm, dsmash is usually the best choice to interrupt it with, since Falco will probably bounce back into a situation he can't quite recover from. Same but less so with nair only less humiliating ^^
edit: If I miss anything or if there are any questions, just add/ask.